I've had to reinstall gmusicbrowser. I have a large library (>100K songs) and would like to access old labels etc. I have 'old' files called gmbrc.bak: 47.3 MB backup file and a number of gmrbc.bak.yyyymmdd.xz files.
I have copied these into the 'new' .config/gmusicbrowser folder, but they are not used. How do I 'force' gmusicbrowser to dip into the backups and start from there?

You can simply rename one of them to gmbrc, (though I wonder why you don't have a gmbrc with the .baks) it should work even if it is compressed (the gmrbc.bak.yyyymmdd.xz are compressed)