If you're making a portable you probably need something to watch it on. (Unless you want to guess what's happening in the game, but I wouldn't advise that) Anyway, this forum is your "Hacking a pocket TV/screen" one-stop solution. Share your experiences and knowledge here.
the gamegear uses digital RGB, you would have to build a complex digital to analog controller board and a chroma decoded(if your system doesnt do rgb), the NOMAD on the other hand takes standard analog RGB so dont go calling people pricks when your wrong ok
After buying all the parts to make a crappy screen work, you end up spending about $80; so the answer to your question is yes, but it's not practical. You can buy a HipGear/Intec for about $20, or if you're lucky like me, as low as $5 (thanks Journeyfanatic!). Even a PSOne screen is in the $40 range and is MUCH cheaper than using a GG screen.