It was common in console design to produce modified consoles specifically for gaming magazines and press to use for capturing high quality screenshots for their content. For instance, Nintendo produced a single RGB version of the Famicom (NES) in the form of a collaboration with Sharp. There's a TV model that has a Famicom board built in and pipes the video out over internal RGB connections straight to the TV, hands down the clearest picture you can probably get from real Famicom hardware. While not targeting magazines exclusively, this system was incredibly popular as you could get crisp screencaps without CVBS interference or many color problems (NES was particularly troublesome with colors as stock PPUs literally generate NTSC or PAL, there's no real RGB going on in there).
Anywho, as time went on this became more of a formal thing, with Nintendo (really Intelligent Systems) for instance making the IS-AGB-CAPTURE as a video breakout system to take high-quality GameBoy Advance screencaps.
Of course just speculation, there are some general Game Gear dev hardware details here:
https://www.retroreversing.com/sega-game-gear-devkit
Indeed there is even a unit listed as being designed for the purposes I've described, but it looks like a dedicated board rather than a modified stock console. Considering it's a Sega module I assume it's not just something someone hacked together with off-the-shelf parts. Do you know anything of the unit's providence before you got ahold of it? It's still possible it was a later unit made for this, non-production hardware tends to look very "industrial" in the earlier lifetime of consoles but then they start making stuff using the stock molds and form factors as time goes on. For instance compare a "NPDP-GDEV" or "AMC DDH" to the later "NR Reader" and even "NPDP Reader" that Nintendo shipped for GameCube dev and QA. The former are large computer-like boxes, the latter look identical to or very close to the GameCube you buy at the store.
