Bear with me as this is probably not exactly as it happens, only the way I see the process. And I haven't dealt with 2.8 since February.
Normally in 2.6, when I save an in progress file, I keep it in .psd format. If I want to see it in game, I'll resave it as a .bmp, then reload the .bmp and select indexed mode then save again. This leaves me with two files in the gimp folder: a .psd with all my layers, and a .bmp ready to be dropped into the skins folder in AH.
With 2.8, You no longer have a .psd, but new gimp format. These files are the only files which gimp will open from the "Open" command. Likewise these are the only files that can be saved via "Save" or "Save as". To make a .bmp you must use a new "Export" command and "Import" to load a .bmp. Sounds simple enough, even if a little more cumbersome than necessary, but here is the big problem: exported .bmp do not display properly in game! There a 2 main issues: first, the bitmap is shifted about 100 to 150 pixels to the right; second, the colors do not display in the correct shades, by a large margin. For example, my RLM 02 that I've been using on my Emils is instead the color of pea soup when in AH.
Had I not found a download for Gimp 2.6, I would have given up on skinning until finding a free version of another software.