Here's a theory why Gambino shooters weren't used in the hit. Even though, it may have been a intrafamily plot, it's possible that it was simply set in motion by Philip Rastelli, who was the incarcerated Bonanno boss around the time Galante was on the street and basically took the position for himself. He underestimated Rastelli's connections and the loyalty guys afforded him on the street. Galante could've been as tough as he wanted to be, but once those who were protecting him realized where the power lied, they went with the power. It should be noted that the weeks leading up to Galante's shooting, Joseph Massino, Stevie Beef Cannone, Sonny Napolitano, Nicky "Nicky Glasses" Marangello, just to name a few, were captured visiting Rastelli on numerous occasions. I don't know how that all fit into everything seeing as, Nicky Glasses was demoted after Galante was killed supposedly for being one of the guys who showed Galante the utmost loyalty during his bid for power.

And I don't think Galante would've defected from Joe, by all accounts he was a loyal underboss.