Chris has some mod info that's heavy on pictures if a little light on info. It does, however, feature a variable resistor for blue separation adjustment. It only applies to early Game Gears, which can be more difficult to find and identify without opening.
This mod schematic is rather intense, requiring the stacking of chips and densely packing in components, but that wouldn't be strictly necessary unless you wanted to keep the original case. Lots of additional info to be had.
Finally, this guy sells a logic chip if you'd rather have a simpler time of modifying your Game Gear. It's small enough to easily fit inside the case and kicks out RGB, composite, and s-video signals. Also, it works with any version of the system.