His runes, summoner spells and masteries are quite subjective. Feel free to experiment which path works for you. Although, I recommend you follow this path first if you are new to Gangplank in order to maximize your damage and utility.
Before, Greater Seal of Armor was equally as effective as Greater Seal of Scaling Health, but now you really need the early game armor bonus to combat the more AD based toplanes like Darius and ?Riven. For now, I advise you to take armor for top, and if you happen to go midlane take HP Seals instead.
Flat AD Quints
Again, just go with damage.
Flash is essentially the best summoner spell in the game! An instant gap closer than can also be used as an escape mechanism is always a great summoner spell! You really need this because of your lack of escape. Sure, you might have great movement speed, but that speed can?t always save you all the time.
Teleport grants Gangplank a lot of utility. It will help you set up ganks, split push, and help other lanes when they are in trouble. Teleport is important all throughout the entire game, and it is as important as flash for toplaners. This paired with your global ultimate can actually scare your enemies from going all-in early.
Alternative Summoner Spells
Exhaust is great for slowing down your enemies and winning trades. The damage reduction is very useful and is perfect either for killing your enemies or running away from them.
Ignite is a summoner spell that secures kills for you. You can grab this if you plan to get early kills or you have a slight advantage from your enemy in 1v1s.