Support for Google Music in omPlayer has been fixed. Please reload GM page twice in omPlayer giving it some time between reloads to download updated script.
If that doesn’t work, you may need to close omPlayer, remove folder
~/Library/Caches/com.mifki.omplayer and try again. Sorry for the inconvenience.
Also, it seems that at least GM All Access now requires Flash being installed and enabled. If you see a message that you need to update Flash Player, go to omPlayer preferences, Advanced tab and check Enable Flash for Google Music.