We have said that we hope to add the ability to import GM files (or specifically, identify instruments from their GM program changes), and we hope to add the ability to play back via GM devices, as this is an important user case for users who want an ultra-light playback preview.
However my point is that GM more generally is of little use an interchange format, and hasn’t been for 20 years. In the 4 years of development this thread has been the only request for GM file support.