Thanks a lot! Upscaling the game does not actually change the resolution of the game, at least not in the logic side, but sadly the camera system of GameMaker Studio 2 makes it very difficult to disable subpixel drawing, that's why I let an optional 1x resolution.
While this of course doesn't look very gameboy-ish, I was actually surprised to learn that the og GB did have subpixel calculation that obviously couldn't be displayed due to hardware limitations. Of course, that's no excuse, but a fun fact