Adding New Brushes
To add a new brush, after either creating or downloading it, you need to save it in a format GIMP can use. The brush file needs to be placed in the GIMP's brush search path, so that GIMP is able to index and display it in the Brushes dialog. You can hit the Refresh button, which reindexes the brush directory. GIMP uses three file formats for brushes.
More info:
http://docs.gimp.org/en/gimp-using-brushes.html
