You may be mistaking Gambino's decision (upon his death) to allow Dellacroce to oversee the Family's lucrative Manhattan operations while Castellano had rule over the rest. Gambino wanted the Family to become more legitimate, and knowing that Castellano had a better head for this type of business he appointed Paul as the boss to replace him. Don Carlo also knew there'd be some factions in the Family that wouldn't be happy with this decision, so he "gave" the Manhattan operations to Dellacroce in an effort to placate Neil.

It depends on which source of information you follow on who was the Gambino Family underboss. Its generally accepted that when Carlo had taken over (after Albert Anastasia was killed in 1957) the underboss was Joseph Biondo. In the mid 60's Biondo did something pretty foolish - he tried to cheat Gambino on some deals - and Biondo was forced to step down as underboss. Carlo then elevated Dellacroce to be his new underboss. Its been sometimes noted that Dellacroce was Anastasia's underboss, and if in fact he was, it was only for a very short time. (That title had been held by Frank Scalise, who was killed a few months before Anastasia).

I have a copy of the Gambino Family chart that the government used in the early 60's (much of the info supplied by Joe Valachi). Both Aniello Dellacroce and Paul Castellano are listed as "Capo". I've never seen any chart which listed Paul as "Underboss".