You should just buy one; there's no way it's going to be more economical for you if you have to ask for help, unless your time is worthless. "Pinouts" are available on the net.
Just as people told you on the SMSPower boards, it'll just be easier to buy one from the open market. Making one by yourself is an option, but a MasterGear adaptor needs to be sturdy. Homemade adaptors (a bunch of wires and boards, no case) are almost never sturdy.
In case you're really hellbent on making one and have found the pinouts, you could go to www.expresspcb.com and download the free CAD tools there and design your own somewhat professional board. The tools fairly easy to use, and you really dont need an in-depth knowledge of electronics. Only thing that might not work out is the board thickness - they only use 2 different pcb boards, and im not sure what the gamegear/mastersystem pcb thickness is off the top of my head. Oh, and its also going to cost you at least $100 (they charge by board area). So id only recommend doing it if you want to learn, otherwise grab one off ebay.
