Hi there, problem with the GBA is, that it is 160p instead of the usual 240p, that’s why it doesn’t look right. The Scanlines get much thicker and spaced this way, and afaik there never was never a game-console that put out 160p plugged into a TV.
Personally i would use a shader called lcd-grid-v2-gba-color in the handheld folder (cg shader).
With a nvidia card that’ll begin to look strange, so you gotta swap the shader 0 with lcd-grid-v2-nvidia.cg (handheld->shaders->lcd_cgwg).
That would give you the gba screen without the console border image. Kinda like you are looking at a giant gba-screen.
Won’t give you scanlines, but i think it’s worth a try.