From the LTC1799 datasheet: 
I guess that the Gameboy runs at 8MHz, so you would need a 12k resistor (or 6k8 in series with a 47k pot) with the DIV pin on GND.
You can do something like that:
But you would need a floating source, or at least some weird shenanigans… Start with a vactrol.
I think your Gameboy is ok with only 5V, you can try without fry it, at least.
Sam did some clever trick for the mounting, check his video.