In new version of GMMP, you can't set playlist folder manually (or I don't know how to do it).
I got music and synced playlists on SD, cause of 128GB internal storage.
But playlists created by GMMP are stored on internal. It seems, that's the reason why sync app can't do a reverse sync with them.
You can only set a custom playlist folder in the Classic version of GMMP as far as I can tell.
The default path is GMMP\Playlists. You need to put this path into the playlist path field in device settings.
But as with Phred, the sync app probably won't have permissions to sync this folder.
edit:
I can't test this out as the app will not even attempt to sync if GMMP is a trial version but you could try the following -
Set your playlists folder to music\playlists
Do a sync so the playlists are copied from musicbee to the device.
Open GMMP. This should pickup the playlists in the music\playlists folder.
In GMMP, delete one of the songs out of a playlist.
Do another sync with the reverse sync of playlists ticked and see if the deleted song gets deleted out of the playlist in Musicbee.
If that works it means you can modify existing playlists and the only issue is you will not be able to create a playlist on the device and have it sync back.