The Game Gear is a handheld created by Sega. Pretty much a handheld Master System with a lower resolution however it has a higher colour palette. It was a direct rival to the Game Boy however it is in color and was not as successful. Also the console suffered from dodgy capacitors so unless they are replaced, it wouldn't work anymore even if they haven't been used.
Note: No emulator supports the Gear to Gear Mode that was used in a few games such as Sonic Drift 2. The only working emulator that supported this feature was the one that Sega used for Sonic Gems Collection.
MEKA emulates the Game Gear and its console counterpart, the Master System as well as support for the SG-1000 and the ColecoVision. It also features a tile viewer, a palette viewer and a memory editor making it easier to rip from games.
Another emulator that emulates the Game Gear and the Master System, with similar functions (graphic viewer, map viewer, and palette viewer) to MEKA. It can directly save currently loaded graphics as an image file under a dedicated "Tiles" folder, ontop of the common quick screenshot feature.
Emulicious is a Master System, Game Gear and a Game Boy emulator that runs on Java (however it doesn't require it to be installed since there is an optional version with the binaries included). It is newer than MEKA so it is more compatible with newer computers and has more accurate sound with less issues. Like MEKA it has a tile viewer, a palette viewer and a memory editor so again easier to rip games however the palettes can not be edited and the tiles are double sized. On the other hand, no forced PNGs are outputted when print screened so more useful for computers with SSD drives and can be set to native size no matter what resolution your screen is.