I took a look at your file and have a few pointers.

1) You seem to be drawing your mountain with the pencil tool. This creates unaliased (hard edged) lines. I think you would have better luck with the paintbrush tool.
2) Your image is RGBA. If this is saved as a gimp brush (gbr) file, it will end up exactly as it looks - i.e. black on transparent, regardless of the paintbrush colour you are drawing with. This is the mode you want to use when creating multi-coloured brushes. If you add a white background and change the mode to greyscale (Image->Mode) then your saved brush will have transparency where the image is white and be fully opaque where the image is black, with a 50% grey being 50% opaque. This brush will also paint in the current foreground colour. It will all be ONE colour, however.
3) When using any brush, if you use it with the pencil tool, it will be hard edged (i.e. anything less than 50% opacity will be transparent, anything more will be solid). That seems to be the issue when you say "it renders as a solid color after I save it as a brush".

Hope those tips help!

-Rob A>