AP Gangplank is very viable. If you build Gangplank to be AP focused, you will need to build the Lich Bane item early on. It will increase your Q damage significantly. AP Gangplank does two things very well. He is able to heal for a very large amount of health, which makes him able to split-push and solo better than AD gangplank. Additionally, his Ult will deal a lot more damage, which is great in late-game teamfights. You need a lot of items to make AP Gangplank powerful. AP Gangplank is more of a late-game build than an early game one. AD Gangplank is still of course very viable and is strong at all phases of the match. If you are new to GP, we recommend you stick with an AD Gangplank build until you are skilled with him.