According to the gameboy schematic, it's 1k.
http://www.freeinfosociety.com/electron hp?id=1449
That being said, why would you do this? Are you just trying to replace a broken volume wheel? I think someone found a stock replacement part and linked to it on this forum at one point...
Actually, I'm building a synth that incorporates multiple Gameboys in at once, so it'll be easier to manage sound adjustments with an external pot, plus one of the Gameboys I'm using has a completely destroyed volume pot. It's mainly just to make things easier.
Thanks for the help though