My understanding is that the tapes were the cornerstone of the case. He talked in such detail, and with such verbosity in that apartment that there was no mistaking what he was saying. At a minimum this was a guy who was aware of murders happening under his watch. At a bare minimum he basically gave them a RICO conspiracy case. The tapes alone were enough to give you say a 98% chance of giving them 25 to life

Gravano’s testimony was more corrobation of what Gotti himself had already said on those tapes. Gravano flipping was just the icing on the cake that turned that “98%” certainty of a conviction into 100%. But without the tapes, Gravano’s testimony could’ve been written off or, at least, discredited to a degree by a skilled lawyer

Gotti, in essence, unwittingly ratted himself out. His voice on tape was the trial’s star witness.