.ABR is a filetype for Adobe Photoshop brushes.
If you want to convert them to .GBR/(I forgot the other one), then you must get explicit permission from the maker asking if you can convert the brushes. If you do get permission, then use ABRViewer to convert the brushes to .PNG. Then open with The GIMP and save it as a .GBR (or whatever it is).
ABR Viewer:
http://abrviewer.sourceforge.net/