Hi,
I added an old version to the downloadable package that has the feature you want!
Install the G.A.S. Old version 3.0.1 from the package, and then you proceed:
- Import the 8 frames into G.A.S.
- Make sure "auto-adjust to include all frames" is unselected
- Set the horizontal and vertical frames to 8 and 2
- Set the last frame to 16
- Now export as "Single Image" to get the sprite sheet with the empty area you need
The only feature missing in this old version is the "Manual Crop" FX function. If you don't need this function this might be a good temporary solution for you before I drop the next big update that will also include empty sprites feature.
Have fun with it!