In my opinion, Gambino has never considered Dellacroce to be a potential successor.

Don't forget that Gambino was very hostile towards the Anastasia faction of the family. Dellacroce was one of the few Anastasia guys he could work with so he made him underboss in order to keep the family united but there was no way, he would make an Anastasia guy boss of the family.

In that regard, Gotti taking over Castellano could be viewed as the Anastasia faction taking back the boss position from the Gambino one.