Any brush you make not using the brush editor is not editable in the brush editor. The brushes supplied by gimp are also not editable.
You can open a brush as an image and edit that and then re-save it as a brush.
According to the manual, brushes created using the brush editor are called parametric brushes and they can be edited. I may have one or two parametric brushes I made with the editor but I have many more that are ones I made as images and saved as brushes.http://docs.gimp.org/2.8/en/gimp-concepts-brushes.html