It tells how to import an Alpha texture using Photoshop, but… ah… I don’t have photoshop… 
How would I go about importing an Alpha texture using the GIMP?
I use the GIMP, so I think I’m qualified to answer. 
All you need to do is save the image as a .psd. Most, if not all, of the image data from the GIMP is stored, and Unity natively imports it automatically. Edit, save, and go.
Also, beware of the alpha-blending bug. When Unity blends the image, it blends indiscriminately. If the background is transparent, it color is blended anyway, normally giving objects a weird white glow. There are prepackaged photoshop actions to fix it, but I haven’t heard of a prepackaged GIMP solution.
Hope it helps.
By the way, you posted in the wrong forum. This should go under “External tools”
Moving it there as we speak