The original Game Boy and Game Boy Color require a Y1 Tri-point screwdriver to remove the screws securing the case.
The Game Boy Advance requires a Y1 screwdriver and a Phillips 1 screwdriver (PH1) to remove the screws on the cover.
The GBA SP requires a PH00 Phillips screwdriver and Y00 Tri-point screwdriver to remove the screws securing the case.
If you're trying to open the original Game Boy's cartridges you'll need a Gamebit 3.8mm screwdriver. The GBA cartridges require a Y0 screwdriver.