As you might know, if you quit SSB through the home menu, the system will restart in sysNAND, which can make playing the game kind of annoying. If you hit home and instead of quitting, run a CIA game from the home menu, the other game will launch, and quitting from there will bring you back to Gateway mode. Since the only way to save a .3ds rom is to exit the game through the home menu, I HIGHLY recommend installing SSB as a CIA game.