As a long-time Gentoo user, I can tell you that it's perfectlly capable of producing both a useful gaming rig and a useful server, provided you have some idea of what you want to end up with.
Proprietary nvidia drivers should be straightforward enough: emerge nvidia-drivers and blacklist the nouveau module (or compile a custom kernel that doesn't contain it). You'll probably want to read what the Gentoo wiki has to say about Steam.
Good luck.