The gmbrc is corrupted. Try a backup, you have several backups in ~/.config/gmusicbrowser
Sample output
% ls -og ~/.config/gmusicbrowser
insgesamt 34588
-rw-rw-r-- 1 12943689 Jul 5 19:33 gmbrc
-rw-rw-r-- 1 12943689 Jul 4 22:24 gmbrc.bak
-rw-rw-r-- 1 1893728 Mai 6 14:54 gmbrc.bak.20150506.xz
-rw-rw-r-- 1 1894384 Mai 7 09:34 gmbrc.bak.20150507.xz
-rw-rw-r-- 1 1895944 Mai 8 14:49 gmbrc.bak.20150508.xz
-rw-rw-r-- 1 1900908 Mai 19 10:17 gmbrc.bak.20150519.xz
-rw-rw-r-- 1 1923200 Jul 4 14:49 gmbrc.bak.20150704.xz
Open a terminal and do the following:
mv ~/.config/gmusicbrowser/gmbrc ~/.config/gmusicbrowser/gmbrc.xxx
cp ~/.config/gmusicbrowser/gmbrc.bak ~/.config/gmusicbrowser/gmbrc
After that, start gmusicbrowser again.
If all this does not work, then you will need to delete ~/.config/gmusicbrowser/gmbrc and start over. Sorry :\
But as little bonus, here is a daily build PPA.

gmusicbrowser in action with some additionally layouts