Gangplank is one of the champions I consider OP (overpowered)
Now you may not think that he is that good but he can actually
send the strongest champions running, here's how...

The skill sequence above focuses on charging towards the enemy and shooting them, in this particular scenario you will want them to hit back unless their attack is greater in that case just heal and run and keep repeating.

The sequence above should be able to damage and kill an enemy (not counting the ultimate) but
if it doesn't, just ult them and continue chasing.

Lastly, Gangplank is also great at dealing with multiple enemies, I have tried it and he is great, I was fighting against Taric and another champion, I started by using "Parrrley" and slowly killed Taric while also dealing melee damage to the other guy (forgot his name[he had a giant axe]) I eventually killed the other guy and used "Raise Morale" to charge at Taric, I was too far away to use Parrrley so I ulted him and killed him.

The items you will need are listed here and above (with what they do [not including unique passives])

Berserker Greaves: 920
Executioner's Calling: 1350
Cloak and Dagger: 1450
Phantom Dancer: 2845
Hextech Gunblade: 3190
Infinity Edge: 3830

All together you will need
13585

I suggest you use Gangplank with Ashe or some other ranged person because if you attempt to assault someone and fail, you may need a ranged to cover you while you run so that you both don't get hurt.

You may also use Gangplank with Amumu. What I do is use my ultimate and let Amumu get inside and use "Despair" and slowly damage everybody while I finish them (KILL STEALER)

There might come a time where you can't get to an enemy which is why you might need "Flash"
Flash lets you teleport to where the nouse is...

Say your right next to an enemy turret and don't want to get damaged, you would use Flash and get past it with little injury, but if you do get damaged, that's where "Heal" and "Remove Scurvy" would come in handy.