There were some "Gamegear only" titles (or sequels of already existing titles) released... There were even some games that have been ported from Genesis/Megadrive and might have unique gameplay and levels or they just have the same name but they are completely different.
Here are the ones I remember now:
-Sega 4 in 1 Pack
-Shining Force 2, Sword of Haija
-Other "Gamegear only" Shining Force titles (Japan only)
-Strider Returns (Not sure if a Master System version exists)
-Sonic Triple Trouble
-Gunstar Heroes (Not sure about that)
-OutRun (Unique gameplay)
-Various "Gamegear only" Shinobi series
-Mickey Mouse, Legend of Illusion
-Power Strike 2 (Unique gameplay)
-Columns (Unique gameplay)
-Fatal Fury Special (Unique gameplay)
-Megaman (Unique gameplay)
-Panzer Dragoon Mini
-Virtua Fighter Animation
-Incredible Hulk (Not sure about that)
-Power Rangers (Unique gameplay)
Don't remember anything else... There are more however
