Has anyone used GIMP to make or alter maps. if so how successful was it ?
Ultimately the imaging software you use has nothing to do with your chances of success, provided you output the correct grey-scale image using the right image format and size, the game will load it.
When it comes down to it, the results you can attain are more closely linked to your skill level with a given app. But it matters not whether you use MSPaint, GIMP, Photoshop or any other particular piece of software.