Just wondering why gangplank is listed as support

That is his second role, it doesnt mean tho that as it is his role he is the support, he is more support because he only kinda helps the team more than kills (wiht ulti, and att damage + speed) all he does of damage is Q and R. That is why he is considerated support as secondary roll.

he can Q minions with targons for gold, and his damage is quite high at lvl 2 or 3 fights. And long time ago he could deny friendly minions with his E skill was pretty fun back then

He is absurdly dominant level 1-2, E max gives your team something like 5k gold worth of stats in mid game fights, works well with Targons, can gank lanes without leaving his ADC?