From Guild Wars Wiki

Warning: Running Guild Wars on a non-Windows operating system, or with any other computer which does not meet the system requirements, is completely unsupported by ArenaNet and NCSoft.

[edit] Preparing your Gentoo system

You must have a working gentoo installation, including working sound drivers (guide) and hardware accelerated 3d graphics (guides for ATI, nVidia). Your user must be added to both the audio and the video group. Make sure your drivers work before trying to run Guild Wars. If you have trouble, try asking on the gentoo forums.

[edit] Installing Wine on your Gentoo system

Run the following command as root to download, compile and install Wine:

emerge wine

Continue on Guild Wars on Wine to complete the GuildWars client installation.