If it makes you feel any better, I don't think there were any champs with reverse control immunity when he first came out. I might be mistaken though
The reverse control immune champs are usually pretty weak in this fight. You are better off getting used to the reverse with a heavy hitter and ending the fight sooner. I used 5* Doom.
I've beaten the GM on 3 different accounts now, and on all 3 I used 5* Colossus.
Most advice here is useless without knowing what champs are on the team. You can't swap them now so you have to make them work. Focus on the champ that does most raw damage and just keep reviving. For me it was Juggernaut, who is immune to reverse controls, but I picked a team for the path and each phase.