From Bryan Cockfield on the Hackaday log:

Game Gear HDMI with SNES Controller

With its backlit color screen and Master System compatibility, the Game Gear was years ahead of its main competition. The major downside was that it tore through alkaline batteries quickly, and for that reason the cheaper but less equipped Game Boy was still able to compete.  Since we live in the future, however, the Game Gear has received new life with many modifications that address its shortcomings, including this latest one that adds an HDMI output.

Here is a video of it in action:

Very early prototype using my GBA HDMI board to get a 1280x720p output from the Game Gear.

The custom PCB uses a Spartan6 FPGA to convert the Game Gear’s 160×144 12-bit RGB video into a 1280x720p HDMI output using a 4x integer scale. HDMI video is generated directly from the FPGA, audio is taken from the Game Gear’s headphone jack.

It has some pixel glitches, but it could be due to the wiring as it’s very sensitive to positioning. The Game Gear was bought as a “broken” unit and is in need of a cap replacement, that could also be causing issues.