Unfortunately I'm not really sure -- Unity 3D, the engine we use, doesn't officially support anything except for Ubuntu. I don't know what differences there might be with Gentoo, and I suspect the problem might be in there.
In the very worst case, you can use WINE to run the windows version of the game should-be-flawlessly on your Gentoo installation. My guess would be there's some sort of sound driver issue or possibly some other middle-layer compatibility issue, but I am not familiar with that linux flavor.
I wish I could be of more help! If there's anything you have that you think might provide some clues, I can definitely take a look.