Gangplank can be really good if you make him, with this build follow what i have done and make sure you always use your Q as a poke.

At LvL 18 with full build, your basically unstoppable and you can 2 shot their adc, unless it's a crit gangplank.

Always try and get last hit, it is sometimes good to get it with your Q because if your up against a xin and ezreal than it is good to just farm using your Q but it uses a lot of mana so you might have to buy a couple of mana pots.

You can help a lot with your E so be sure to use that in a team fight and also use your Ult whenever you can.