I have had this issue for about 2 months now. I have 100 percent completion bar, and all acts are checked as completed....and it is not unlocked.
So I figured I would go through 4 player co-op because I might have missed something. I got the achievement for that, and the mutator after about 5 hours today....but no my fellow gears achievement. I know a lot of people have this issue, and I do not know if there is a solution floating around out there. However, I would appreciate some feedback as to what I should do. I just want my achievement