Sorry about that. Automatically downscaling the window has caused all sorts of confusion, and I'm not sure if it's even necessary anymore. I added it as a hotfix for some version of Windows, where windows larger than the desktop were completely unusable (Windows always force-minimised them).
I've created an enhancement ticket here:
https://github.com/gosu/gosu/issues/341Also, Gosu 0.8+ should use software fullscreen (like video players...), does it not work for you? Which version of SDL 2 are you using?