I downloaded sameboy from retroarch menu but it doesnt show in the core selector on batocera game settings. Is it possible to add it to the core list for gbc games?
Yes because this is impossible with Batocera.linux. If you want to add a another core your need to add the core on the source code and compile your own build
You can check the source here: https://github.com/batocera-linux/batocera.linux