I can't find anything that specifically mentions converting PNG to GBR, but I'm sure it can be done. In this tutorial it shows an image with a transparent background being made/converted into a GBR brush:
Turn a Texture into a Brush Easily in Gimp!

After you've used abrMate to convert Photoshop brushes into PNGs, you should be able to open those PNGs in GIMP and then follow the tutorial from this part: "Our New Brush, Ready to be Saved"