This is my Gangplank build.
As I enjoy playing GP I found a build that works for me, and I decided to share it.

For marks I go with Greater Mark of Critical Chance for the early game crits, you can go go armor pen if you want.
Seals Greater Seal of Scaling Mana Regeneration.
Glyphs Greater Glyph of Scaling Cooldown Reduction.

Quints 2x Greater Quintessence of Revival and 1x Greater Quintessence of Movement Speed. Same here, armor pen if you want.
With the 21 points in utility, that's -20% time dead, and +4.5% movement speed.

9-0-21 Masteries
I take 9 in offense for the extra 15% magic penetration in Archaic Knowledge (helps with the ultimate) and 3% extra cooldown.
And 21 in utility for the -10% time spent dead, and the rest of the utility masteries, like +5% champion experience gain from Awareness and an extra 6% cooldown reduction.

So I start with 9.45% cooldown reduction. At lvl 18 without CDR items it will be 17.62%, with boots 32.62 and with Youmuu's that's 47.62%(before Youmuu's 42.62%).
Even though it is over 40%, it helps early game, before you get your CDR items, or you can sell the boots late game for merc treads, and be capped at 32.62% CDR.

I start with Boots, 1x Mana Potion and 2x Health Potion.

Next items are the 5 Gold/10 sec. items.
These are important to get early on as soon as you have the money.
That will be 800 for the Philosopher's Stone, and 750 for the Avarice Blade.
A total of 1550. If you got an early kill/assist and farmed enough with your Q, you should have these pretty fast.

If you didnt had a great start, go buy the Philo Stone, as it helps with your mana problems, and keep farming that Q. Then get the Avarice.
Those 2 5 Gold/10 sec. items + the utility mastery + Parrrley's passive, means you will be a beast late game(if you also farm).

Next are the Ionian Boots of Lucidity.
If you farmed enough or got a few kills, get the The Brutalizer, else just farm some more then back again and get it, but don't upgrade it to a Youmuu's Ghostblade yet.

Now that we have mana / 5 sec, some crits(and 5 gold/10 sec.), CDR and some armor pen we need damage.
The next item is going to be the Infinity Edge.
We are going to start that off with a B. F. Sword, farm some more. Then Cloak of Agility, farm some more. Then finish it off.

Now we are ready to finish off that Youmuu's Ghostblade too.
You should have enough to buy it after the Infinity Edge, if not, farm some more and buy it after.

Next we should build into a Phantom Dancer or Trinity Force, by buying first a Zeal, for the extra attack speed, critical chance and movement speed.


Now that the game dragged into late game, we need some survivability.
We can achieve that doing the following.

If the enemy team has a lot of magical damage, we are gonna rush that Banshee's Veil, or Force of Nature(I prefer B.V. as it synergizes with the next defensive item). If not so much magic damage, just buy the Negatron Cloak first(we must have some magic resist after all) then some armor with the next item.

In a balanced team, there should also be some physical damage.
Atma's Impaler is right for the job, giving armor, critical chance, and increases your AD based on your health(thus the B.V.). Start by buying the Chain Vest.
If the enemy team has some hard AD carries(Yi, Tryndamere, Nocturne...), and if they focus you in every team fight, Atma's won't cut it, you will need a Thornmail, like the Atma's, first buy the Chain Vest.

Now finish either one of the defensive items, I prefer to finalize first the Banshee's then Atma's. Your choice, depending on the enemy team.

If you have the spare money, you should always get a pot, either an Elixir of Agility or an Elixir of Fortitude.

If the game really drags along, this should now be over 40 or 50 minute game, sell the Phantom Dancer and get a Trinity Force. Not more you can get now...

I start leveling first Parrrley for the farm, and it is my first spell to maximize.
Because of the extra gold it gives, it helpes a lot. Even if you solo lane, and have the same creep score with the enemy, you will still have more money from its passive, thus winning your lane.

My next priority spell is Raise Morale, as it gives AD and movement speed.
If you are being harrased and are afraid of being killed, you should maximize Remove Scurvy over this.
Or balance them both.

Of course we upgrade the ultimate, Cannon Barrage as soon as we can.

R > Q > E > W or R > Q > E = W

For summoners I take Ignite + Flash.

For a long time I was going Flash + Ghost.
But I found out that ignite was exactly what was needed for the early game kills.
Crits + ignite + Raise Morale + Remove Scurvy ends in a sure kill, thus you being fed and winning the game.

Flash is a must. If you get focused or are in a bad spot, Remove Scurvy + Raise Morale then flash. Clean getaway.

You should be farming Non-Stop.
And spam that Parrrley, you shouldn't have mana problems, except maybe before the Philo Stone, but that's to be expected.

Use your ultimate to farm creeps.
Either to protect a tower if it is being attacked or if there are creeps there but no allies champions.

Use your ultimate if you see one of your lanes is pushed(after the outer towers, this will make someone go there) and there is a huge minnion wave incoming. It's on a pretty low cooldown with 40% CDR, so dont worry.

Just farm farm farm.

Also important are jungle creeps. If you dont have anything to farm, or to do, just do the jungle.
Get the Lizard Elder(red buff), as it helps alot.

At the end of the game, if you don't have the most gold or most creeps, you didn't do a very good job at it.

You need to buy Sight Ward watch out for ganks, and set up ganks. Or ask from your team/jungler for ganks.
Early game 2v1 GP + a jungler is enough to kill almost anyone in your lane.

Scan the battlefield, and use your ultimate to help your team out. You will either get a kill or an assist out of it, no problem :D

Use Raise Morale in every team fight, it helps to give that extra boost your team might need.
And use that Youmuu's Ghostblade. Place it in the #1 or #2 slot and use it!!!


Don't be afraid to fight.
Parrrley your enemy from time to time, or auto attack them if they come close to last hit.
You need to learn GP's limits and how much and who he can fight. You can only learn this by fighting...

If I see the enemy stays and fights, I'll Raise Morale and start auto-attacking + Parrrley.
This will make most enemies back off. If they still stay, that's when I'll maybe ult(if I'm over lvl 6, this can be done pre lvl 6 also) + Ignite, then chase them to death.
If they flash to get away, and you think they will die to Parrrley, then flash for it, it might even crit and secure that kill...

Mid to late game, after you have bought The Brutalizer and maybe some parts of the Infinity Edge, you should definately roam and gank the lanes. As you are very powerful and should help your other lanes.

Haven't tried this in ranked, because I don't play ranked.
But why wouldn't it work.

If Gangplank wasn't banned, since they always ban him in ranked.

Feel free to post results from ranked games also.

I really like to use the utility on all of my champs.
Like the less death timer I find it really good, instead of some more armor penetration. And some movement speed.

"Why not get armor pen runes or Last Whisper?"
Well I wont be targeting the tank, and most of them wont have so much armor that the Youmuu's Ghostblade wont suffice. Never had a problem with armor on carries or squishes...

"Why not buy a Frozen Mallet?"
His new passive slows enough, and dying shouldn't be a problem. Don't get cought and flash if focused( Remove Scurvy + Raise Morale should be engouh to get away, if your team is nearby).

"Why not rush Sheen?"
I don't like rushing to a Trinity Force, since it is the last item I get, and it will take space for a long time. Even though some might say that it is good, I just dont like it.

"Why not go 21-0-9, or other 21-9-0?"
Like I said, I like my utility masteries, that's how I play most champs.

This is my Gangplank build and how I play him.
This has proven very successful, for me at least.

Remember, if your team does not lose early game(die excessively, lose too many towers), with this build you can't and shouldn't lose mid to late game...

You can't win them all, at least not alone. You still need a decent team, a team that will listen to you when you tell them what to do, or who to focus, a team that wants to win. You can't carry a team that does not want to win.

Screenshots from the last days with this exact build(solo normal queue).

My last games:

Full builds:
Note that I didn't buy the Trinity Force in the full builds, even though I should have. Get it if you don't know what else to buy.


Patch:
This guide was made with the patch v1.0.0.121 in mind.
If they change anything, I'll update it.

This build is not easy. You must be constant on the look out, farm and help your team.
Ward and get different objectives on the map.

But it pays off, if done right.

Thanks for reading :)


PS: Excuse any mistakes I did, English isn't my first language ;)
PS2: This is my first build, so please be gentle :)