Other Game Gear heavy hitters:
GG Aleste 1 & 2 are extremely impressive 8-bit shmups. Fast, fun and easy to get into.
GG Shinobi 1 & 2 are very solid action platformers, especially 2. They set themselves apart from the console Shinobis with mild Metroidvania elements (permanent upgrades, returning to past levels to access new areas) and multiple playable characters with different abilities.
Ninja Gaiden is nothing like the NES trilogy, or the Master System game - it's basically more like Shinobi in pocket form. It's a short game, but I like it a lot.
Defenders of Oasis is a nice turn-based RPG, starts out like a 16-bit game though unforunately there isn't as much story after that.
For fan translations, there:
Zenki - Solid action-platformer developed by some of the GG Sonic team, with some non-linear elements as well.
Shining Force Gaiden III: Final Conflict - another solid, pocket-sized dose of Shining Force
Royal Stone - nice little SRPG with nearly 16-bit level graphics
Moldorian - cool turn-based RPG with good pacing
Last Bible is a fantasy Shin Megami Tensei spin-off, better than the GB version. The story is clearer, the colors look great, and there's a new bonus dungeon at the end. It feels like battles and everything move faster too, making the game more pleasant to play.