A GIMP brush is a brush for GIMP. GIMP stands for GNU Image Manipulation Program, is free and has lots of features.
The answer I know is this
1. open the gbr file using Gimp, save the pic as png format,
2. open the png file with photoshop, read this link [link]
, following tutorial instructions on how to make photoshop brush with pic.
Hope it helps, if not 2 years later.....