So I'm a total newbie, and I'm having issues with my brushes going on sabbatical after awhile, but I'm thinking those are peculiar problems to my situation and not gimp in general.
Ironically, I ONLY know how to make color brushes. Simply put, I create what I want to be my brush on a transparent background, then when it's exactly what I want, I select the area I want to be the brush, and copy. Then I go to the Edit menu, down to the Paste As option, and one of the options is "Paste as new brush" A window pops up to allow you to name your new brush, and you name it and voila, you now have a brush that works in technicolor. (as in, whatever color you made your brush in will be the colors for your brush forever.)
So if you have a white background, you'll have a white background when you use the brush.
There are more complicated ways to make brushes (and I only know one of them), but this simple method works.
Word of warning... if you make a couple hundred brushes, you might start losing them like me, so personally, if I was doing it all over again, I'd not only choose the "paste as new brush" I'd secondarily choose "paste as new image" and then save the new image in a folder as a back-up.
Hope this helps. I'm getting some awesome advice in the Case-of-the-disappearing-brushes, but thought I'd help with probably the one single thing I am (mostly) good at.
