Hi i just want to say you something about GP, i have a lot of experience with him.
Why chooose Gangplank- A lot of fun with parrrley. When you crit you can get some small ****** when you will do half hp by one spell. I think its pretty strong champ with mallet and lifesteal he cannot be owned in 1 v 1. So strong, fun is what you can find on GP

Marks- I using Armor pen. its best choose what you can do. It boost your dmg well.
Seals- Crit chance is best here. You cannot take better seals. Why dont take dodge or armor telling me some guyz ? I just reply WTF why these runes. Its **** you dont need about 6% or 7% in dodge or 10 armor. Crit is what you need Crit make you OP. GP is crit master
Glyph- Same like seals best option it makes me to have 16% on start + i have a lot of armor pen.
Quins- Best is Armor pen. its like Marks. In my option its best for most DPS champs like GP

21-0-9

That's your choise! 21 is commonplace for DPS, and 9 its good for GP i tried it 21-9-0 with improved dodge change but it didn't helped me. I like to have improved Flash and some regens is good too.

I taking Braw. gloves form start which make me more stronger and dangerous i continue to avarice blade then for brav.gloves. (First stop in home with 1270 minimally)
Then i taking Cloak of agility for Crits which is most important. Then i continue to finish Infi. Edge which is best item what can Gp have. 250% crit dmg is very very needed. From now you are killing machine.
Then i taking Phantom dancer. Makes you faster and again a lot of crit chance + att speed.
Then i taking Frozen Mallet. Slow + Lives is deadly combo.

Ok so like in my guide name Parrrley is primary what you want. Remove scurvy. Its good for laning and PRIMARY for defuse of stuns slows or taunts which is best for runners like veigar or from rammus which can be your dead when rammus taunt you in teamfight.
Many people ask me why i didnt lvl up raise morale. I think it is useless in early game. In late game its good for move speed or dmg. I think there are better spells to lvl up BEFORE raise morale

So ignite is your choise for getting down running people.
Ghost is best for GP it makes you come back in lane or catch some runners.
I tried exhaust too but it dont helped me a lot like ghost.

In early game pick up your Braw. gloves and 2x health potion. Taking down creeps by auto-attackin and try to harras your enemies with your primary skill PARRRLEY !!
Mid game. Jungling is good choose what to do with GP when you have slow buff you slowing by your parrley which is important thing to get easy kill.
Late game. Dont start teamfights try to have both buffs in teamfights. To reduce cds and slow. You can kill casters in 3 sec easily which make you most DMGer in team so you will get focused much times.