According to Jerry Capeci and Gene Mustain in their Gotti book:
"If it turned out Gotti was not personally involved in heroin, he (Castellano) would, despite his misgivings, make Gotti one of the three 'acting street bosses' who would run the family". The other two were Tommy Bilotti and Tommy Gambino. Gotti was not thrilled about this. In Gravano's book he says that Tommy Gambino would be named acting boss if Castellano went away.

I'd guess that Gotti would have gone along with being underboss to Gambino's acting boss, but really be the one running the show. Based on the Castellano tapes, it doesn't sound like Gambino was too keen on being a mobster, let alone a boss. So he would be the boss but really in name only. Then he retires or goes to prison and Gotti becomes official boss.

With DeCicco it's a whole different story. I don't know if Gotti would be able to get the support to move on him, or even want to.