Yes. That theory has started to grow on me too. I don´t remember ever reading Johnson having any connection to the Gallo gang, but he was once employed by a sleasebag who ran stripclubs for a Gambino associate, if I´m not mistaken. This seems to be the closest Johnson ever came to a mobster. I don´t think I´ve ever read he was in prison with Gallo, at least it doesn´t ring a bell with me.
But he was close to the black couple who worked at a party (mentioned above by Scorsese), held by a guy named Russo, in NJ or Staten Island, I believe. This Russo was a Colomboo associate and became furious when he saw the black waiter dancing with a white woman at the party. A fight broke out and Russo killed the the black couple.

I have been trying to find the article describing the events, but can´t remember where I last read it. If Johnson shot Colombo as a revenge killing, then the Colombo hit was not Mafia related at all which could explain why no informant knew anything about it or no researcher or police investigation has come up with any substantial leads or any shred of evidence in this case. They all seems to have been looking the wrong way.