there are several games that are pretty good rpgs on gba.if you want gameplay footage i suggest looking at youtube to get a general idea
pokemon ermerald/ruby/saffire-Some might not consider this a rpg,but when you dissect the gaming element piece by piece it's a rpg.Pretty straight forward game you capture pokemon with pokeball,earn badges and battle your opponents.Very little has changes have been added in previous series,the new additions include tweak game mechanics to be faster pace such as hotkeys,added pokemon,two vs two pokemon match and new designs for older pokemons.Still a pretty fun game to play with when link up with a buddy
fire emblem-Very entertaining srpg that uses the triangle system or rock,sissor paper style.Your weapon has some dexertity and will slow diminish with each use.This game requires lot of patient.Never rush you character recklessly (espcially for boss fights that stands on high elevation-use opposing weakness to counter it)or your character dies and when they die,there gone for good.Down side is limited xp gain.Most can be gain in colleseum
fire emblem sacred stone-More user friendly compare to first one.More xp spots and more money spawns
final fantasy 1+2/iv/V-It a final fantasy game,enough said.It still has that old school charm with addition to new modes and graphics.Watch out when turning combat system to max speed you can get turn glitches,sometimes you can get 2 straight turn(seriously i think its a good thing)
megaman battle network series-good game,but squel are almost identical copies
other good ones im too lazy to write about
super robot taisen 1+2
tactics ogre
final fantasy tactics advance
rebel star commander
mario and luigi superstar saga