A photoshop .abr file can have many brushes in the file. Many, but not all, .abr brushes can be used in Gimp. Depends on the photoshop version used to create the .abr file.
There is an old plugin that can open a .abr file in Gimp as layers. Then export as a .gih
Attached is the 32 bit version (13kb exe). There may be a newer 64 bit version, but I don't have it.
EDIT: Forum won't let me attach a .exe file. So I have put it in Dropbox
https://www.dropbox.com/s/cyzahn6pf0aja8m/abr.exe?dl=0
Forgot to say
The .exe goes in your plug-ins folder
There is an old plugin that can open a .abr file in Gimp as layers. Then export as a .gih
Attached is the 32 bit version (13kb exe). There may be a newer 64 bit version, but I don't have it.
EDIT: Forum won't let me attach a .exe file. So I have put it in Dropbox
https://www.dropbox.com/s/cyzahn6pf0aja8m/abr.exe?dl=0
Forgot to say
The .exe goes in your plug-ins folder