A GameBoy Color version of Jet Force Gemini was in development by Bit Studios (for Rare) in 2000, but it was cancelled in the end. The original JFG is a third-person shooter / action adventure published for the Nintendo 64 in October 1999, but the GBC version was never officially announced.
We were able to know about this cancelled project only thanks to Dano2k0 from the Assembler Forum, that found a playable prototype and shared some screens with the community. Rare released 2 other GameBoy Color games based on their Nintendo 64 titles, Perfect Dark and Conker’s Pocket Tales, that were received with low interest by gamers: we could speculate that their third GameBoy Color “N64 port” was canned for this reason or for quality issues.
Jet Force Gemini GBC was going to have an isometric 2D view, while the gameplay was probably going to be “similar” to the N64 version, with lots of shooting and insects to kill.
As we can read in an interview with Martin Wakeley:
JFG on the Gameboy was the only occasion I can remember Rare outsourcing anything. It was being done by Bits Studios and was nearly done last time I saw it, I’m not sure what happened to it.