Just my opinion,but I think Gotti knew that a plea deal by any of his guys would include an elocution in open court. This could include the admission that the perp was a member of the Gambino Family. In Gotti's mind,if there was no Gambino Family,then he couldn't be the Boss of it.

One of the common sayings by defense attorneys was "It has never been proven in a court of law that the Mafia exists".
Of course the Commission Trial changed that,but for what it's worth, Gotti at least had some basis for his position.

Too bad he was a hammerhead when it came to pretty much everything else.