Description
This is mainly for my own reference since I don't make brushes I need to reuse that often so I always forget a step and have to look it up.
Go to file -> new. Open with advanced options with Color Spaces set to Gray Level and Fill with: white.
Export the brush as a .gbr file to where your bushes are saved. (Find this by uses right click on a brush in the brushes tab and going to show in file manager.)
Refresh brushes to use the new brush.