I'm assuming this happens because GP's Q is made to proc like an auto attack but it is still a spell and AP Gangplank is a viable option, especially with Luden's Echo.
**As stated in the title, Luden's Echo does not activate on Gangplank's Q. **(This makes his ult the only form of activation for Luden's Echo)
Its not meant to proc q, thats why ap gp doesnt exist unless you go crit get 20 kills then go ap.
you are correct that that's why it doesn't. it functions as a basic attack not as a spell.
http://leagueoflegends.wikia.com/wiki/Spell_effects
Luden's Echo will not work with on-hit abilities. The above is why.
One thing that has been discovered is that runeglaive's passive does proc luden's which may make ap gp more relevant than it was, but at the same time there's always other ways to build him that are proven to work.
