First of all, there's a build created by a guest equal to this one. It was me that created it, but forgot to log in... I have enjoyed playing Gangplank solo top for a while. During my time playing LoL, I've tried a great variety of builds for GP and, without a doubt, this one is the one with the best results so far. Even though you have a poor early game (few criticals in early game), your late game is one of the best in the whole LoL universe.

For runes you're gonna need critical strike and critical damage because, without these runes, this build will be useless. You can buy armor runes too, they are normally useful in early levels.

For items, as you can see above, you start the game with brawler's gloves and an health potion. The next time you return to the shop, you should be able to finish avarice blade and boots of speed. After that you start building Infinity Edge with Cloak of Agility (it gives you critical chance, the purpose of this build). After that, if you have the gold, always opt for B.F. Sword instead of Pickaxe. When Infinity Edge is finished, you buy Ionian boots (great for cooldown reduction, which you'll need because of Parrrley (q). Warmog's is what comes next, since you'll be off tank and, therefore, you need health. Atma's gives you AD, armor and critical strike chance, being a great item for you to purchase after warmog's (the so called atmog's). You end your build with Trinity Force.

Since you're going to focus on killing your enemies through criticals, Parrrley is always the best option. Although, if your lane phase ends soon, Raise Morale is of the essence. Remove Scurvy is the last to max, since you will only use it for the cleanse ability it gives you. Using it only for the health is a bad option, as it has a medium cooldown and uses o good portion of your mana.

For your summoner spells, bring flash and teleport. Flash because it's always useful. Teleport because it gives you a great advantage in solo top, since, if the enemy character has no teleport, you'll be able to return to the lane fast enough to outfarm him.