just for the true dmg on turrets it isnt worth playing GP as support .. when u can hit the tower as a melee every other support champ u would have played would have done also some dmg to the tower ...
they question is what u can offer your adc as a support .. gp has no cc ... gp has only self sustain .. gp is heavy dependend on gold income ... playing him top and mid on a solo lane takes already in this meta to get relevant when they games last only 20-25 mins ...
sitting him bot as support makes no sense .. he will not get any gold at all and his gold from the q denies only farm from your adc ..
so whats your idea behind playing gp as support .. what can he offer and do better than a lulu, nami, thresh, sona, braum, alistar, leona, zyra etc??