There are many tutorials about how to convert Photoshop brushes to Gimp (abr to gbr) but information about how to go from Gimp to Photoshop (gbr to abr) is scant. However, there are many who used the freeware Gimp until they could afford a copy of Photoshop and want to transfer their brushes. Or, maybe you have found the perfect brush set, but it's for Gimp and you own Photoshop.
If you do not currently have Gimp, download it at: http://www.gimp.org/downloads/
On your desktop, create a folder labelled "Brushes to Convert." In Gimp, open the .gbr (Gimp brush file) you want to have converted. Access the brush and tap once on a new, transparent background file. Save this file as a .png and place it in your folder. If there are multiple brushes in the set, repeat this with each brush as individual files.
Open Photoshop and access your brushes menu. If you have brushes open, it will save with your set, which you don't want. Select all your brushes with CTRL-A and press "Delete" (this will not permanently delete your brushes, it just clears your pallet).
Open your .png file(s) in Photoshop. Go to "Edit > Define Brush." This will bring up a window with a thumbnail of your brush and an option to name it, but naming your brush is not necessary. Repeat with the other files, if applicable.
Go to the window you would normally go to for adjusting the size of the brush. Click on the right arrow, and select "Save Brushes."
Enjoy your new set.
If the brushes box doesn't appear, make sure you have it selected under the "Options" menu. The greater the resolution and size you put your files in for Gimp, the better quality your Photoshop brushes will be when you size them up large.