How to install:
Launch GIMP and go to Edit > Preferences > Folders > Mypaint Brushes and see where it looks for them, put the custom brushes in that folder.
Alternatively, click the button "Add a new folder" and give it a new path to search for the custom brushes(s) and check the box next to it to activate. (no need to restart)
Blend
Blending brushes to help with painting with a mouse device
Download through download-directory.github.io: here