Many people would not choose Gangplank as support because he haven't got any strong CC, but they forgot how he is absolutely annoying when poking with his Q, and they forgot that he is the champion that start with the highest base health and that he gives a big AD steroid to his team and that he have a Global slowing skill, that could help the team taking a kill and gangplank obtaining assists.
Not to forgot that his QPARRRLEY will proc the Face of the Mountain on hit if he last hits, that helps he farm better due to the passive of his Q PARRRLEY and the ADC farm better due to the Face of the Mountain .
And he is a pirate, that would be enough i think.