It's late so I will keep this somewhat brief. I have added a decent UI to the file scanner so everyone should be able to get their music into gmmp. The first thing you should do is go in and edit the scan paths (there is a button in the scan page) and be sure its only pointing to places with music. I accidentally tried a scan just on /sdcard/ and something caused the scanner to crash. The paths set there are used for the file browser as well.
The file browser itself is not complete but it "works". I need to sort the lists and finish populating all the information in the list. Be sure to run a scan once before attempting to play a file from the browser, otherwise it will probably crash or give unexpected results.
One last neat little feature I threw in is to play a random song on playlist completion. If enabled when your active playlist is finished, it will start playing a random song.. so the music never stops. Enable that through the playlist options.
Edit: It appears if you scan with "delete database before scan" checked it will crash. So erase the data through the android options if you want to clean the db.