First time, I'm Spanish so I say sorry for my English. I need to say thanks to Sepillos, who help me with this guide, the items and the masteries. The masteries are not failed, I'm not a troll. I think this guide can help you with Gangplank, I love it.
Other think I need to say: Gangplank is a carry and a helper, but in level 18 you can steal a lot with Cannon Barrage, but it is use to help your team or run away.

I put that runes because I think:
Mark: helps you with your damage, this is basic.
Seal: with it you can spam your Parrrley.
Glyph: Parrrley will be spawned with it and the Seal. The other team will call you a hacker.

I have chosen these masteries because GP needs lots of critical hits and critical damage, regeneration and armor&magic-resist. With the runes and the masteries you can stay in lane a lot of time.

If you want to follow my order of items you can, I always do it and I always win the game.
The last item is always a problem, you can try some items like Bloodthirster, Youmuu or The Black Cleaver.

I have chosen these summoner spells because they'll help you going away or running to kill someone.

You only need to focus in one and do auto-attacks or your Ultimate to have a guaranteed death.

To win some "extra-money", when you are alone in lane and you know you are going to go base, you can use a lot of times your Q and farm a lot with a prize of 12g.

I add a picture of my last games with GP

I want you to try this build and comment what do you think about the build.

Regards,

Luzon

European Server