The whole point of GURM is to either add or extract brushes from the brush directory in your user files.
You do not actually place any brushes in the user/gimp 2.6/brushes folder.Move all of them to the GURM folders you created.Then you can add or subtract them from the brushes dialog at any time.
If you want a smaller set of brushes you have to catagorize them to a dif folder in that GURM brushes folder.
For instance mine is set up like this -
C:\GIMP\all sub folders here one is called brushes, and inside that folder are all my catagorized brush folders clouds/spatter/galaxy, and so on.This way when i add brushes i add only the set i want.
So simply put create more folders in your Brushes folder and place brushes of dif type in those.Then call those with GURM.
Hope that helps a little.
Indecently make sure only default brushes are in the program directory share/brushes folder.
Just an added note - do this with your scripts also if you can.
What i did was alphabetize them.I know which letter the script i want starts with so i call it like that out of it's alphabetized folder and that brings only a few scripts in the menus.That works for me until i categorize them by what they do.My first step for that was to find them in the menus (ie add the name to the help popup in the script "something.scm" so then i can see that script in the menu - see its name and now i know exactly what it does and where to catagorize it.