There's a man named Gangplank and he's the swarthiest pirate that ever lived. He is capable of crushing almost every top laner, and he will never be messed with by any kind of patches because Rito hates fun/pirates. All in all, he's incredibly safe. There are only three matchups i can think of where GP get's reliably forced out of lane. At the end of the day, he is guaranteed to be equal on farm, have more global presence and a ton of free assist gold because of his amazing ultimate, and his farm is always guaranteed to be worth more than his opponents.

But let's be real here. This is Gangplank. Whatever skills he has as a top laner won't translate into team fights in any manageable way, and ultimately you have to rely on your global presence and manly lane dominance to make your team strong enough to carry you. Let's just say it's no surprise that I'm still silver 2.

GP is most amazing because he's a million champions in one. He's essentially a blank canvas, and how you decide to skill him or build him will make him entirely different. It's important to know when to max what skill, because depending on the matchup each one is viable.

Maxing Parley.
Parley is a fun skill, you shoot people and they die. When they die you get the mana refunded so you can last hit forever. Ultimately, it's an auto attack on a stick. The more you skill it the more extra damage you get, and the more parley gold you get for each last hit. The latter is the most important part. If you plan on maxing parley, then you need to take full advantage of that extra gold. Obviously, Parley gives you a large amount of burst damage. If you're up against a champion who has a ton of burst damage (like say, fizz) then you need to level this skill.

Maxing Oranges.
Recently Oranges has been the cool new thing to do with GP. If you look at the numbers, you can see why. The heal if maxed is ridiculous. Unless you have a lot of burst, there's no way you can throw a GP with maxed oranges out of lane. Makes for excellent dive escapes. Unfortunately, it's also impossible to pressure your lane opponent. You won't have any damage and if you don't buy some kind of waveclear item like tiamat or statickk, you will lose your tower. Do this against opponents that you know you can't beat. Consider buying Tear or chalice depending on whether your up against AP or not because you WILL run out of mana very quickly. Surprisingly effective against Riven if you build Iceborn first. You can actually kill them.

Maxing Raise Morale
This is the man skill. If you want to duel your top lane opponent, you MUST max this first or you will not be able to keep up. It's an invaluable steroid and the extra passive damage boost is sorely needed with GP's poor scaling. While parley is all about burst, this skill is about the sustained trades and chasing. The longer a fight goes on the more this skill is worth it over parley. In top lane where duels last a while it's important that you max this skill first. Not only will this secure kills in top lane, it will also help your entire team in the mid game, where raise morale combined with a well placed ult will make the opposing team feel like it's running through molasses in comparison to yours.

One rule of thumb, if you don't max Parley first, consider maxing it last. The biggest strength of the skill is it's snowball potential with the extra gold and the ability to chunk your lane partner into oblivion once he gets behind. You won't be able to snowball without maxing it.
That said, if you don't feel like you need any healing then feel free to max oranges last.

Much like his skills, there are a million ways to build GP. Most of them are just based off common matchups. The biggest differences in how you build GP are based on how you will utilize the parley's on hit effect. Iceborn, triforce, tiamat, statikk, or BOTRK (pretty much, only choose one of these).
Eventually, you will need to buy some tank items like Warmogs, bashees veil, and randuins if you want to stay alive in teamfights. Infinity Edge is almost core in most games. GP is a rich man, he can afford it. If you ever see yourself with the chance to buy BF sword DO IT.
Don't worry too much about building damage in the early laning phase. Most of the time, your passive and a dorans blade should be more than enough to kill your enemies.
AD Assassins: GP's best matchup. IF you start cloth five they will run out of mana before killing you and you will be able to all in them. You have a most excellent build path into iceborn gauntlet. Even coming back to lane with tabi and glacial shroud will give you kill potential and tons of mana for bully parleys.
Yasuo is the prime example. Yasuo can do nothing against you. If you max raise morale he won't be able to run away from you and you will get free kills. Every yasuo builds full damage, which is wasted against your early shroud and tabi, and is actually stupidly squishy. They never expect the GP damage. Your passive is strong enough to kill them.
Pantheon is the big exception. His shield will block your parley's, and his spears are better than your parley's to begin with. Even if you get out of his stun, by the time you run away you've already walked through 3 ticks of his E...

Against other strong AD: Riven or fiora, max oranges and build Iceborn. They are difficult matchups because fiora can reflect your parley's, and Riven can shield them and are even better at the early all in thing. Try to play it safe and farm well. Fiora is hopeless while Riven is more just challenging.

Against AP: Vlad/Ryze/Rumble/Teemo.
Teemo and Ryze are especially difficult for GP. Both of them have low CD pokes that have more range and in some cases more damage than parley. You either need to establish lane dominance early by forcing an all in (ryze and teemo are both pretty squishy at levels 1-6), or switch to a new lane. They will poke you under tower, and they will force you out of farm.
Vlad and Rumble are both doable matchups. Building Hexdrinker and merc treads are the best way to go. You're probably going to want to get Botrk if you want to duel them or some waveclear like statikk or tiamatt to avoid losing tower to how fast they push.

Super tanks: Chogath, maokai top.
You can't kill them, and you can't get them out of lane. Even if you get an early kill, their sustain/tankiness means the best you can do is farm and hope for the best. Obviously get botrk.

Jax: Super common these days. He's a ***** though. Early game is free reign, and killing him in early fights shouldn't be difficult. You need to orange out of his stun, and save parley until after his invulnerability is gone. If you can fully apply your passive then it should be no problem. I like to grab cutlass against jax and then move on to trinity force. or wardens mail. Eventually you will need something to lower his attack speed if you plan on dueling him. Most jax's these days grab teleport. Punish this with ignite and go to town. Make sure that jax fears the pirates wrath.

GP loves this stuff. His level 1 all in is the strongest in the game. You have to remember the slice/shoot/slice technique. If you parley right after an auto it resets the animation, making for an amazing amount of burst and three stacks of your passive. That and ignite should net you an easy level 1 kill if you catch them by surprise.
Wait in the top lane bush in river if you are on purple side. If they face check the bush after walking to lane from helping their jungler from blue buff then you've already gotten first blood.
You can do the same on blue side with the tribrush, but it's not as common for them to walk that way unless their team started red buff, and it's easier for them to escape that way...
Watch out doing this with jax, if he starts stun then you need to run away from him once he uses it and you cannot attack him or else you will lose. Might want to consider starting orange for that purpose alone.

You've either gotten first blood or forced a back. If you have, congratulations. Snowball into something beautiful by maxing parley or raise morale. If you haven't, don't sweat it. If you're behind then Get some pots, conserve mana by only last hitting with parley when you have to, and stand under tower. They will die if they try to dive you and you have oranges up. They can't make you leave so just free farm for the rest of the game. You're farm is worth more anyway so you win the lane. If you're just level then try to go toe to toe with him early on, perhaps when you get your ult or right before they get theirs depending on matchup. If you come back to lane with a shiny new item don't be afraid to kill them with it. Not only does your ult force a fight, it also does a surprising amount of damage if they get hit by RNJesus.

It's important to remember to believe, and to be a man. If you engage on most champions, they will burst you then walk away, hoping to win the trade. This will be scary, because their burst will be stronger than yours. Your faith will shatter, you'll shake at the knees. You will want to run away, but if you do that you will let them win the trade. We can't have that. Especially when you max raise morale, your strength as a champion is in your ability to stick on them with your passive slow and your extra move speed to make prolonged trades. All you really have going for you at that point is extra movement speed and way stronger autos.
So when they go up to trade with you, let them get that first burst in. Let them feel like they will kill you. Then when your passive stacks are on him and his burst is done, you've remembered to be a man. You've remembered that if you keep attacking him then your sustained damage will be way stronger than his burst, and the only limit is how well you can stick onto him (making phage super strong).

By now you've won the lane. Your team has given you lots of assists and your ulti has won the game for you. If you're smart you've roamed a lot. Now you're moving into the mid game. This is GP's big power spike. If you bought IE it's time to 1 shot the enemy carries and then auto attack their nexus a lot. If you behind just peel or splitpush. Most of the time you can be a great initiator with your ult and raise morale, and you can choose to get super tanky too. Either way, GP feels like the pirate king of the world.

Let's be honest though. This is solo que. This is GP. You're team has done jack **** while you have been getting kill after kill in top lane. No amounts of well placed ults could help your other lanes. Now you're simply fighting uphill. The enemy team's team comp is probably better than yours because despite your awesome raise morale and ult ability, once you get into the chaos of a team fight your going to melt and only have the time to fire off 1 maybe two parleys. GP functions great in skirmishes/duels but terribly in team fights. If you have the perfect engage you should win no problem. Too bad that's not going to happen.

GP's greatest strength is actually his ability to reliably be ahead on gold against his opponents. That's literally how his parley works. His stats are designed around the fact that you will have better items than your opponent. The sad part is, once you get to full build, you have until they get to full build before you have 0 chance of winning.
I feel that the best way to win is to maximize this window of time by making your full build much better than theirs. This is why slot efficiency is way more important to GP than other champions. If you invest in an Infinity edge, you will fall off much slower than if you bought statikk as your crit item. (That's why I don't suggest statikk unless your going hardcore splitpush).
It's also important to lead your team to take objectives, and win the game before it reaches those stages. GP's power spike is more akin to Pantheon than anyone else, which is where a lot of people's misconceptions spring from. If you can't get other people in solo que to get together and win, then you will lose.