Bock's the man to ask to be sure, but I think it's just a mode that some GG games were programmed in that just uses the standard SMS palette rather than the extended GG palette. Probably done to allow dead easy ports.
Basically it means the game is a SMS game fit into a GG cartridge. A dozen GG games are actually SMS games (same resolution and color). The Game Gear has full compatibility with the SMS so it can run those games.
The reason it is mentionned in MEKA is because as a result when you play those "GG" games you get a SMS-sized screen and it may be curious to some people. MEKA and a Game Gear just see them as SMS games.