Playonlinux

Playonlinux is an excellent frontend for wine, a windows emulator for linux, that will allow you to play most Windows-based games under linux with comparable performance with equivalent hardware sets.

At the time of this writing, playonlinux is a masked package, so in order to install it you'll need to unmask it via portage or by configuring the following entry:
file: /usr/portage/package.keywords/unmask

app-emulation/playonlinux ~amd64

Once unmasked, emerge the software.

$ sudo emerge -av playonlinux

After the emerge, run the software. Wine versions can be managed via Tools->Manage Wine versions. Install the latest version of x86 wine, and you should be able to install a game.