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Since it's possible to play SMS games on a Genesis with a PBC or the modern mini one, is there a way to play Game Gear games on a Genesis?

Given their inherent similarity to SMS games it seems like it should work if there were some sort of adapter...

Nope. You can't pull that off with the Mega everdrive either.

Closest we got to it is the Retron 5 has a new game gear adapter that plugs into it's Genesis slot and requires a firmware update to work. Didn't hear if anyone tried using that adapter on a regular Genesis to see if it works without the firmware needed

Will not work without firmware.

To play Game Gear on TV:
Retron 5
RetroFreak
Some other GG emulator on a PC (or whatever) hooked to television
Modded GG system with TV output

Regurgitating this from what I've read, feel free to correct:

It has to do with the color palette. The GG can handle the SMS, but not vice versa.

There's an adapter for SMS on GG:
https://youtu.be/cnLxyeXHaV4

The Genesis literally has the SMS hardware inside & the Power Base Converter is a pin adapter. So SMS will work, but not GG due to restriction mentioned above.

Chris Covell hacked a few GG roms palettes so they can be played on a SMS, or with an adapter a Genesis. http://www.chrismcovell.com/segahacking/gg2sms.html

I was just about to bring up the hacked ROMs. There are a few conversions to SMS out there, so that's about the only way that will work.