I'm also trying to have a 2 way sync running for playlists with GMMP and I'm facing another issue. Not sure if it's related to what psychoadept and thezyth are experiencing...
Basically, MB to GMMP works fine but weirdly, as soon as a change is initiated from GMMP, the reverse sync causes a lot of empty duplicates on existing playlists renamed with HTML ASCII encoding and other duplicates with "playlists" as a prefix. Those duplicates don't exists before I run the MB Wifi Sync app or before I click Preview. Furthermore, empty playlists are copied in but changes are not actually reflected in MB.
Synchronise all music + Synchronise all playlists
Playlist storage gmmp\playlists\
Save as format M3U
GMMP Playlist conf: UTF-8
MB Playlist conf: Tried both MBP and M3U just in case it would change something
MB Wifi Sync conf: Sync from computer as configured in MusicBee / Reverse sync to computer playlists only /gmmp/playlists / from player GoneMad
Is it a config mistake or a misbehavior?