search on youtube you have to find a folder(!)- not the installed programme- called "gimp 2.6" or whatever version you have. If you click on that, there should be a lst of other folders like "brushes", "patterns" and so on and after you downloaded(or opened the package) you can just copy the brushes into that folder Oh and that gimp folder itself should be found in another one with the name you have on that computer, if you understand what I mean
Specifics: I belong to a site called Aywas where you can make custom pets. I would not be making any royalties off of using this brush but it is technically commercial use. If I were to sell a custom pet I would make site currency - 'BP' or 'GP' but that's the extent.