Depends on what you mean by “edit”. Most Gimp brushes are pixel based. Ones created in the Gimp brush editor can be dynamically resized during use…you might even be able to alter the shape as well…I haven’t tried that. I have mine set up so that if I press the shift key and roll the mousewheel I can scale the brush up or down. Again, this only works with brushes created in the built in brush editor (You will need to adjust some settings in the preferences menu to enable this, see the FAQ on gimptalk.com). Everything else, you basically have to create a new brush in Gimp itself and save it out as a .gbr file, either in the Gimp brushes folder, or in a folder you have created yourself and pointed Gimp to to look for brushes. You can also save brushes out using a selection and script-fu (Script-Fu>Selection>Selection to Brush, which saves the current selection out as a brush and makes it the current brush).