$20 is well worth it, well I think so anyway, also if you have enough for an SP you could just get a regular GBA along with the GG. There are planty of Great games for Game Gear:thinking of picking up a GG with the SMS convertor for around $20
Shinobi 1 & 2 (Although not true to Shinobi Gameplay these are fun Ninja games)
Shining Force Sword of Hadyja (well however it's spelled)
Sonic games all 15 of them (ok so there were, I think 7)
plenty more too, just depends on what games you like!