Gangplank can crit with his q or his Parrrley which does more damage then your basic attacks so it will help more sometimes you can end up critting for more than 900. It also helps with basic attacks.

I take Flash and Exhaust on Gangplank because Flash is good to either get away from any sticky situations or get close to an enemy in order to kill or maye hit your ither spell, exhaust, I take exhaust because it allows you to keep someone from escaping you so you can pick up a kill, it can help you get away, or it can help you in a 1v1 by reducing damage to make up for Gangplank's squishiness.
Reasons for not using some spells-
Ignite- Your Passive lets you apply a poison that already does decent damage when you hit with your basic attacks and as well gangplank usually uses alot of burst damage by critting meaning if you hit someone later game you usually dont tneed damage over time.
Heal- You already have a built in heal
Most other spells are spells not usually used but these would be the most common replaced spells.
Some Alternatives that would work well-
Ghost- Similar to flash helps get away and catch up to enemies