How to add grayscale value to alpha channel

[Skinny]

The grayscale value of the alpha channel in my DDS file determines the extent to which the body of the car, in my racing game, reflects the sun.
How can I assign a grayscale value, varying from white to black, to the alpha channel in Gimp?

[Ofnuts]

To edit the alpha channel you add a layer mask an edit the mask. When you "apply" the mask, it becomes the alpha channel.

[Skinny]

Many thanks !