#1 He is a pirate...this in itself is more than enough of a reason to play GP.
#2 He gets to eat oranges and everything is K.
#3 He gets to shoot stuff.
#4 Did I mention he is a pirate?

Pros
He can win his lane by not giving up kills and keeping up with your opponents farm.
Has excellent ranged harass and makes squishys cry.
Very mobile and he brings a free quicksilver sash.
Global ultimate you can use to help other lanes.
Strong base stats and he scales well into late game.
Able to farm under the tower effectively with proper use of parley.

Cons
There are many champs played regularly top that are much stronger than GP early on.
only 1 damage ability and reliant on auto attack damage.
Not an incredible lane pusher.
Lacks the all in dive ability (Gap Closer) that many bruisers can use to get to squishy champs.

GP isn't a prototypical bruiser as he lacks those balls deep abilities, ideally you want to have a team that can poke with GP as he can poke at the enemy team prior to a fight in the hopes of giving your team an advantage. GP is at his strongest at the mid game as he will generally have a good gold advantage and items to match if he has done his job. Take advantage of your painful poke and move around the map as GP has excellent movement speed and will be able to cover much of the map quickly.

Please mouse over the portion that says "notes" as I have attached notes to all of these sections.

There isn't really a secret formula to GP and his abilities, they are pretty simple, it is all about maximizing their potential.

Passive=Grog Soaked Blade: Gangplank's basic attacks apply a poison debuff for 3 seconds that can stack up to 3 times. Each stack deals 3+(1×level) magic damage per second and slows the target by 7%.
This ability is basically red buff, keep in mind when you parley or auto attack you apply this ability and it deals extra damage. This helps you stick to people as they try to run.

Q=Parley: Gangplank shoots a target enemy with his pistol, dealing physical damage. If it is killed with this ability, he gains extra gold and refunds half the mana cost. This ability can critically strike and applies on-hit effects, including his passive.
Physical Damage: 20/45/70/95/120(+ 100% AD) Extra Gold: 4/5/6/7/8
This is what makes GP who he is...his signature ability. Use this to poke and to last hit. This ability is the only way to keep in a game where you are not able to trade with your opponent as they are winning the lane. This will also keep you ahead in the lane if you are winning. It applies on hit effects so this build is built around it and that ability. Use it to poke to chase and to allow to the ability to escape. Couple this with an Iceborn gauntlet and it is a chasing/kiting machine.

W=Remove Scurvy: Gangplank eats a large amount of citrus fruit, instantly removing all crowd control effects on him, as well as healing himself.
Heal: 80/150/220/290/360(+ 100% AP)
Eat and orange and it's K, this is your get out of jail free card, use it wisely. The way to judge a good GP is based on how he uses his oranges and his ults. If you eat oranges all the time then you are either losing really badly or you don't know how to play GP. Save this ability as it is designed to save you. Also be careful not to use it early, some times waiting a moment to use it will save you.

E=Raise Morale: Passive: Gangplank's attack damage and movement speed are increased permanently.
Passive Attack Damage Bonus: 8/10/12/14/16 Passive Movement Speed Bonus: 3/4/5/6/7%
Active: Gangplank fires into the air, disabling his passive boost but inspiring himself to gain additional attack damage and movement speed, with allied champions within range receiving half that amount, for 7 seconds.
Active Self Attack Damage Bonus: 12/19/26/33/40 Active Self Movement Speed Bonus: 8/11/14/17/20%
This ability is why GP doesn't need boots early and he will continue to scale without damage items early. Use it as a steroid to take objectives with your team or as a team fight erupts. It is an excellent chasing ability and/or a getting the hell out of dodge ability.

R=Cannon Barrage: Gangplank signals his ship to fire a cannonball barrage upon a target 600-radius area for 7 seconds, granting vision of the area and slowing enemies inside it by 25% until 1.25 seconds after they exit the area. 25 cannonballs randomly drop over the duration, each one dealing magic damage to enemies within a radius of 150.
Magic Damage Per Cannonball: 75/120/165(+ 20% AP)
You can not count on the ability as damage as it is inconsistent, it is however a slow. Placement of this ability can make or break a fight or gank as proper placement can be the reason a gank succeeds. Some of the other uses are that is it global and can be used to clear a tower that is being pushed by minions, and it gives you presence in other lanes during a fight.

GP is actually a very safe champ to pick as he has really good sustain and the ability farm under the tower. There are plenty of champs that "counter" him, but counter is a relative term. Many of these champs rely on a stun to then fully combo their target. GP has oranges and if he uses them correctly he can negate this ability to some extent, this also make it a questionable call to dive a GP as he can kite you around a turret. GP's oranges also allow him to use them as sustain. GP can also use his oranges to get away from ganks especially against champs that rely on a singular CC to initiate the gank. GP doesn't have a gap closer/exit ability so when you decide to fight with GP you are basically committed to the fight at that point, This is why poking is very important for GP as you want to make sure you will come out ahead in an engagement. Now for the most important thing in relation to GP. Use your Q to farm, this is your advantage. With a Maxed Q and an Avarice Blade GP's Parley kills give an extra 10g. This is basically a free minion every 2 parley kills. This is why GP can be a very safe pick as you have the potential to out earn your opponent no matter the match up.

GP shines against most AP tops in my opinion as he can out damage them early. For example GP counters Vlad hard early as Vlad is unable to get away from GP in an all in engagement. Most of the APs that play top rely on a stun for them to maximize their damage and GP is able to remove that. This is why I think GP does well as a counter to many mid champs. Once GP is all in you better hope you have flash or can out right win the fight because you are not getting away. GP does very well against a Nasus as well as he is able to remove the effects of his Wither, and can harass Nasus at range. Ideally you are using GP to shutdown and bully these champs, outside of those few champs GP can be successful if played carefully against all champs as he brings a high level of sustain to the lane. GP will shine against champs that scale best in late game and have a weak early game.

GP has a simple and unique kit that can be very successful in all situations if used correctly. As this is my first guide please leave some helpful notes so I can improve upon it. Thanks.