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".
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.