I’m running GIMP 3.0.2 on a Mac M4 Mini running Sequoia 15.4.1. I have downloaded some custom brushes, and put them into the folder specified in the GIMP settings. I have verified the folder several times. Unfortunately, none of them show up in GIMP. I have closed and reopened GIMP. I have tried the Refresh Brushes at the bottom of the Brushes panel. I have closed GIMP and erased its cache and reopened.
Can anyone suggest what might be going wrong?
caveat, I use linux 3.0.2 appimage
Although you have already checked the brush folder location several times, it is still worth checking in Preferences → Folders → Brushes for the actual path that Gimp 3 uses.
…but it is most likely the type of brush. Are these PS something.abr brushes ? Old types work here, (I think type 5 and earlier) but any recent PS brush containing actions and other PS specific stuff are not supported.

I actually have three different types of brushes, all of which I got from sites claiming these were GIMP brushes: .gbr, .csh, and .ai. None of them are loading.
The .gbr is a correct suffix for a Gimp brush - might/might not be a valid file. .csh and .ai are not brushes .ai is a PS Ilustrator vector image.
Gimp native brush formats are .gbr - a raster brush .vbr - a parametric brush and .gih an animated brush + old style PS brushes
Edit: Hopefully a Mac user will be able to help.
You could try a little experiment to prove the Gimp path-to-brushes. Make a little image, export as a brush to your brushes folder, Refresh the brushes. Does It show ?
quick example: https://i.imgur.com/VMf0z7t.mp4
Interesting… I created a brush as directed in the video, put it in the brushes folder, refreshed, and… nope it doesn’t appear either! Which at least tells us it isn’t a file type problem.
EDIT: Apparently it’s possible to use .csh with some manipulation.
EDIT: I deleted all the non .gbr files in the brushes folder in case they triggered some error in the refresh process. Refreshed again… still didn’t get the custom ones.
SOLVED: In the settings for brushes, I selected “Open in File Manager” and lo and behold there are TWO Library folders, one visible and one hidden. When I moved the brushes to the hidden one they loaded perfectly.