I’ve created this tutorial whit GIMP 2.2
To load /install /add your brushes to GIMP you can have your
application open, just copy your .gbr brushes files (I recommend
copying, but you can move the files as well) to GIMP users
default folder. It is usually ‘C:\Documents and Settings\’your
profile’\.gimp-2.2\brushes’ created during the installation process.
GIMP doesn’t read the subfolders, so remember to place all .gbr files in
main brushes folder.

Your new brushes should now appear in a Brush Tool menu,
after clicking a brush tip icon.

If not, try to open the Brush Selection dialog, and
click the ‘refresh button’. It should work if you placed your files
right.

If you still can’t find your brushes, got to File>Preferences,
choose Folders>Brushes and check the exact location
of the brushes folder.

Maybe you somehow placed your brushes in a different location? If so,
move them into the proper folder GIMP currently uses.

Second Way
You can also, instead of searching for the default folder, use
another way to add your brushes to GIMP.
Just place your personalized brushes anywhere you want on your
computer (ex: Your desktop). Then go to File>Preferences
in GIMP menu, then Folders>Brushes (just like above
), click on ‘new’ icon, choose your brushes
folder location and then confirm by clicking ‘OK’.

This way your folder is added to GIMP and your brushes should be
visible after GIMP restarts.
Once again: Be sure all your .gbr files are in the main
folder not in subfolders (.gbr files are brushes files for GIMP 2.3 and
lower, so Photoshop brushes – .abr files won’t work with those versions
of GIMP!
The second way is usually more handy for all operating systems. So
instead of locating default folders, you can add a new folder with the
new brushes in the gimp preferences.
How you install brushes in GIMP 2.4?
You still use the same ways to load brushes in new GIMP version

1. Paste them into default brushes folder and refresh, or…
2. Add your brushes folder in GIMP preferences and restart GIMP
What’s different? You can place in your brushes folders both: GIMP
.grb files and Photoshop .abr files. They work in GIMP 2.4 and they are
scalable! Now you don’t need to convert .abr into .gbr But still remember not to place
files in subfolders: they won’t be seen.