Wasn't Castellano basically running things for Carlo the last few years before Gambino died? Should of been obvious i would think that Big Paul was his guy.

I also agree with what Ivy said that Gambino probably believed Castellano was the better choice to take the family into more profitable white collar crime and legit activities. And actually i think Paul was the better choice at that time. Too bad Johnny Boy didn't like it but thats the way it was.