0.25 will add the ability to edit the tags of your music from within gmmp. The tag editor can edit individual tags or do mass tagging for certain fields like genre, artist and album artist. You will be able to access the tag editor from the following views: playlist, artist, album, genre, song, and browser.
Using the tag editor is fairly straight forward. Check the boxes you want to edit and the text box will become enabled. Any tags that are not checked will be untouched.
This release is taking a bit longer than I wanted it to but if all goes as planned I'll have it up for download tomorrow.