"Reputed" means claimed or believed to be. A "reputed mobster" is someone believed to be a mobster or accused of being a mobster.
"Fellow" means in the same group or category. How specific that group is depends on context. Often, as here, it's spelled out: "fellow reputed mobster". Both men are in the group "reputed mobsters". There is no indication that they are in the same gang. You are trying to make it too specific.
You certainly could say, "fellow members of the Gambino crime family". In that case you'd be saying that they're in the same gang.
In a totally different context, if I said something about Mr Smith and then said, "his fellow physics professor, Dr Jones ..." I'd be saying that both are physics professors. Nothing more than that. I am not saying that both teach at the same college or have anything in common other than that they are both physics professors.
The two men MIGHT be in the same gang. Or they might not. The statement as given doesn't say.