I've had Gangplank for quite some time, and I'd love to see a little work done on him. However, I have no idea how that should be done...
As a solo top, he gets beaten down pretty easily, and he'll waste all of his mana if he uses his Q to harass (meleeing the typical solo top is nearly out of the question due to him being so squishy). He can become somewhat tanky via items, but this really cuts into his damage, and tankplank will never be as tanky as a more common solo top, such as Darius or Garen.
As jungle, he's very squishy and difficult to keep alive during his first clear. Vulnerable to counter jungling because he isn't fast and is likely to die if an enemy comes in on him. His ganks are also sub-par because he has no hard CC, only two slows.
Gangplank can go mid if he can dodge skillshots (depending on who he's against). This is generally not a good idea because he will be outranged. His W can keep him safe from the hard CC that many mid characters employ.
AD carry is what GP was designed for, according to his tags, and he can do it competently. However, this is uncommon because he's melee. He does have a steroid, a free cleanse, and two offensive abilities... Honestly, I think ADC Gangplank could be high-tier if more people tried it, but again, melee adcs are difficult when laning against typical ranged ones, and they are looked down upon.
Supportplank is on the rise. Personally, I love playing as supportplank, because if you get him pickpocket, he becomes a second AD carry that doesn't need to kill minions, ever. He allows his carry to farm freely by suppressing the enemy adc, which can lead to easy kills for GP's carry. His only flaw for this role is that he can only give his adc a speed/AD boost. No heals, unless he takes summoner Heal, but I think he's better served taking Exhaust. He works rather well with AD carries with dashes/blinks, such as Vayne, Graves, Ezreal, and Corki. His W also allows him to absorb some damage for his adc and sustain himself, but he has to rush Philo stone (or Tear of the Goddess/manamune) to keep his mana up.