Hmm...
I think the problem you might be having is the fact that it's ABR file type, which is mainly used for Photoshop, and Gimp doesn't really work with them. Try downloading one thats a GBR file like this one:
https://dev-moon.deviantart.com/art/kami-Hair-Brushes-for-GIMP-356543116
Once you have downloaded and unzipped them, open up gimp and go to GIMP> preferences. At the bottom there should be a little folder icon called folders, click the drop arrow beside it and more options should appear. The first one that appears should be called brushes, click on it to find all of the brushes you have loaded into gimp, there is also a button (an open folder) that lets you upload GBR brush files to that list, allowing you to use them as brushes in gimp. Select the files you want, wait for them to pop up in the list, then tick them and click ok.
That's how I did it on my Mac anyway, and I can go into more detail if needed. But I'm not an expert and it took a lot of google searches and YT videos to actually learn what to do.
And I believe you can get those ABR files to work in gimp if you save them as a PNG file somehow, and them open the up in gimp and save them as a GBR or something.
I hope this helps! and Good luck!
Morning Showers then Cooling
