I don't think they were based on the Gambinos because in one episode when Tony and Jonny Sac are talking about taking out Carmine Lupertazzi Sr. Jonny says that if they whacked Carmine in a way that didn't shed light on them it wouldn't be like what happened with "Castellano" - invariably referencing Paul Castellano's murder by Gotti. Jonny actually says "Castellano" in that episode.
Maybe someone already pointed this out...too lazy to read all the posts