These are so expensive nowadays! Curious question, do you know if the Gemini is compatible with the Supercharger? I think it would be easier to play games that use all the buttons using the Gemini since all the buttons are on the front of the console.
I cannot say 100% but I do not know why it would not be, unlike modern clones using emulators the Coleco Gemini seems to be a hardware level copy of 2600 hardware.
I found only THIS info searching, it does not mention the Gemini specifically (it mentions it does not fit the Coleocovision 2600 adapter) but considering the Gemini slot seems wide enough to accommodate the Supercharger I am guessing it would be fine.
The Supercharger is compatible with Atari 2600, Atari 2600 Jr., and the Sears Video Arcade consoles. Due to the shape of the Supercharger, it would not normally fit into the ColecoVision's Expansion Module #1, which is an adapter that allows the ColecoVision to play Atari 2600 games. However, if the cover of the expansion module is removed or an extender is used, the Supercharger will work. Extenders were sent to customers who called Starpath about such issues.
The Supercharger did not work on many Atari 7800 systems (which is typically backward compatible with the Atari 2600), although it does with some early models of the system. After Atari installed a circuit to fix a compatibility issue with the 2600 version of Dark Chambers, it subsequently caused incompatibility with the Supercharger and some other games that use the FE bank switching method."