The TV tuner has an AV input jack on it. It's a 1/8 headphone jack, so you just need an RCA to 1/8in converter, and then you can plug the AV out from a VCR or cable/dish tuner into the the game gear.
You could make your own Nomad using it.
Just get a Genesis 3 and a UPS (battary back-up). Use the UPS to power the Genesis (my 1250VA UPS powers 2 computers and a 19in monitor for 45 minutes, so I figure it should power a Genesis for about 10 days straight.). You'll put that in a strong backpack.
Then duct-tape the genesis 3 to the back of the Game Gear, and use the AV cable + converter to plug it into the tuner.
And viola, you've got a Nomad.
Hmm. The Genesis and Game Gear use the same AC adapter, I wonder if the Car Adapter for the game gear would work on the Genesis. Then if you were in a car, you could even do without the UPS.
But I've always thought that would be cool to do one of these days. Go on a camping trip with a fully powered 1250 UPS in your backpack... roughing it as they say.