I want to say it was running Retro Pie? Edit: I don't have it with me, but I think it was the mGBA emulator, sorry.

That's not an emulator, it's a frontend. You'd need to know the core, e.g. mGBA or VBA-Next.

RetroPie is a collection of emulators (and also distributed as a special-purpose Linux distro), mostly ported to the libretro API, and all being accessed through the emulationstation GUI.

For GBA emulation, RetroPie has the choices of "gpSP" (either through libretro or independently), "vba-next", and "mgba".

edit: Sorry, jpfau, saw yours after refreshing.