After all this time of getting reports of GMMP not showing up as a "default music player" I was able to fully understand what the issue is. It wasnt until recently that someone specifically mentioned google now + default player (the first part was always left out, so i made the assumption it was the default player when opening files), so I started playing around with google now's "play X" voice commands. GMMP did not show up there and after a little research into the voice commands, I decided it would be worth the extra day or 2 to add that in before closing out the 2.1 beta.
So now you should be able to select GMMP when doing voice commands like "Play Artist" or "Play Song By Artist" or "Play Album By Artist"
2.1.0 Beta 6 (10/07/2016)
-Fixed issue where sdcard write access is not saved when using 'save to playlist'
-Fixed crash when saving new playlist
-Force left align for text from tags stored in a RTL language
-Backup stats will now save ratings in the tags if they are not already set
-The trial expired / invalid unlocker message pop up will now launch straight to the unlocker in google play / amazon when okay is selected
-Improved parsing of synchronized lyrics
-Added google now voice command support for playing artist, album, or song
-Fixed crash during song transition