Gangplank

The best keystone masteries seem to be Thunderlord's Decree and Grasp of the Undying. It might be difficult to proc Thunderlord's Decree with barrels and bullets (you need to auto attack), but your ultimate can quickly proc Thunderlord's Decree all on its own. And Grasp of the Undying still applies on Gangplank's Q, which applies on-hit effects, giving him more healing in lane.

(My personal favorite keystone for Gangplank is Stormraider's Surge, but since exploding his barrels already speeds up Gangplank and slows down anyone he hits, the speed boost from Stormraider's Surge is going to be overkill nearly all of the time.)

Also, going with Thunderlord's Decree lets you get 45% CDR and the Bandit mastery, which gives you 10 gold every time you shoot an enemy. I recommend this path.

You'll want to use AD runes. Lethality runes are an option, but are weaker early on.

I recommend 10% CDR from runes, also. If you want to use all flat runes so that you have 45% CDR by the time you've finished two items, that'd be 9 flat CDR blues and 1 flat CDR quint. That's probably best. Reds and the other quints are probably AD, and yellows could be hp/lvl or armor or mr.

Your general item path would be You can get Mortal Reminder if you need Grievous Wounds. You might sell boots in the end for another zeal item, like Phantom Dancer. If you don't need a Last Whisper item (because nobody on the enemy team is building armor), then you could get another zeal item or maybe Essence Reaver so that you have 100% crit chance.

I got most of this from watching Tobias Fate.