Has this been posted up yet?
It had few pointers that eventually helped me around with alpha and gimp enough to actually have some effect, but my lowly skills are perhaps not enough to get most out from it, also I think that version used in page had many things rather different that what I have in my gimp version.
I have GIMP 2.7.1, I can't remember what reason it was, but I had to install dev version as 2.6 had something that made something impossible that I needed to do, it is so long ago that I really don't remember now was it even related to alpha channels, but it had something to do with either astro imagery handling or textures of rfactor, sorry that my memory is so poor these days.
There is now 2.7.4 already available, so I might upgrade to that.
I can't afford Photoshop either and ultimately I would love to use Blender for my 3D needs too, but at the moment it is quite distant dream, maybe some day.
My method to do Alpha in Gimp might be wrong one, but feel free to say if it is bad way, I try to describe it here as well as I can, it is very difficult for me however, so bare with me.
First I remove any existing alpha layer from image, I do it from layers tab, right click small image of layer and choose delete alpha channel. It must be done as if I edit existing alpha it just adds it to existing alpha and I get wrong results when alphas are piled up over each other.
Second I add layer mask by right clicking layer choosing layer mask, then choosing alpha channel.
Third I right click added layer mask and choose show mask, then I do my edits to that layer mask.
Fourth I right click layer mask from layers tab and choose apply layer mask, this creates the alpha channel to image.
I never directly edit alpha channel as I have not found it to work, or I lack skills of making that work.
Every time when I want to edit alpha channel I must redo that.
It surely is possible to hide other channels and to have only alpha channel and edit it directly, but it seem to make different results.
As I'm not graphic artist, all terms, ways, methods are unknown to me, so that makes it quite difficult to describe what I do, also it can be very wrong what I do.
One annoying effect of this edit of mine is that texture is almost invisible after applying layer mask, but it works fine in game, if I manage to make it to right level of course.
Oh and I hate alpha, but that is probably because I don't understand and I am not able, anyway maybe this gives some ideas to test and maybe you can find path that leads to desired results, if so I would love to read from it so I could learn more