GBitmap::extrudeMipLevels bug?

If the image format is Alpha then bitmapExtrude never gets called. I am using a gray-scale png as a gloss map and it only displays properly when I get close to the model.

Is this a bug or do Alpha formatted GBitmap objects not use the mipmap? If they don't then why is the bitmap's numMipLevels member being set to 9?