The one and only Jerry Capeci.

Personally, I don't believe it was just a lone nut. It could have been Gallo, but surely he must have gotten permission from the top. Maybe he wanted to make a statement to the Colombo hierarchy that he wasn't somebody who you should fuck with. Therefore he could have ordered Colombo to be assassinated at the meeting.

But I also think it could have been looked like that hit came from Gallo and that he was just the decoy/pawn.