There could be some reasons behind it seeing as getting a death M only requires speed whereas the TGM1 GM would require you to not mess up badly before certain parts and stack cleanly. While I feel closer to death 400 than TGM1 GM it's mostly because my 20G stacking isn't too great.
@JBroms I'm at ~350 on Death, but S7 with game cleared for TGM1. @Archina and I got the same problems apparently, and I can't make tetrises in 20G. As Death is pure survival, it doesn't really matter.
You are much closer to TGM1 GM than you are to death M. It probably took me 7 or 8 months to go from ~350 to M. 400 is yet another dramatic speed increase.