Back Track: Deathmatch Mode.
Back Track is a FPS for the GBA, as far as i know it wasn't really a very "well known" FPS for the GBA but i personally remember owning this game back when the GBA was first released and i thoroughly enjoyed it, In some ways it reminds me a lot like Doom but i'll let you be the judge of that :)
Sorry for the lag in sound halfway through the game =/
Side Note: This game was also available for Windows PC in the late 1990's.