Here is the wiring diagram for the PRS-Tech Gameboy Zero Cart board
Gameboy connector shown still soldered to the gameboy pcb to make it easier to see where the pins are. the gameboy connector card edge connector must be removed from the gameboy PCB before soldering the wires to the Pi Zero
Keep the wires from the card edge connector to the Pi Zero as short as possible
Solder ground and 3.3V wires directly to the Pi Zero board. Do not connect them through a power hub board.