So i just vs'd a gangplank as susan

Yeah, it can be a pretty annoying matchup. It's not so bad if the Gangplank rushes Trinity Force or Statikk Shiv since, unless you expose yourself to autoattacks early, he won't really have the mana to poke you out with a Cloth5 start. However, if Gangplank builds Iceborn Gauntlet he has enough mana to constantly harass you and the slow-field (+passive) to kite you out.

It's not so bad during laning phase unless Gangplank gets ahead, but it can be extremely hard to get to enemies during teamfights. Gangplank can easily use his ultimate to pressure the map without leaving lane, which means he can help his team get ahead while still harassing you.