I have tried some of those settings on my Euro GT50 and I can’t get 3x or 4x to work. That’s not to say those mode can’t be gotten to work, just that I haven’t been able to work them. It’s correct to say that the gt50 does not support pc modes other than 1920×1080, 1280×720 and 640×480.
Other than that the gt50 is great with lots of consoles in 2x mode. With 0.76 pal PS1 games work with slightly tweaked 288p setting but with 0.77/0.78 those tweaks no longer work for some reason