I think that if Antonino Gaggi would not have been indicted and died in prison, that Gotti would have been hit and replaced with Nino as Boss.

My opinion is that Nino as somewhat of an old school Sicilian had solid backing from the Sicilians that used to run the Gambinos before Gotti (not a Sicilian), and also possibly Sicilian Bosses from other families. Nino was adamant about the Gambino family being a Sicilian run enterprise. Anyone who remembers what Nino said to Roy Demeo in Murder Machine agrees. Nino told Roy that only Sicilians can become Capos in the Gambino family, and that was at the moment Roy got made. So Nino was clearly upset about Gotti, a Neopolitan, rubbing out a Sicilian Boss without majority consensus from the Commission authorizing him to do so.

No, I think Nino and Gigante and many others would have eventually gotten rid of Gotti and installed a Sicilian Boss, probably Nino himself. When Nino summoned Gotti to a meeting, he was sending a signal to the Gambino family that he was the new power.