Yeah I was gonna post distrowatch
Phoronix is good for benchmarks. You'll see a range of differences between GUIs such as Xfce and KDE. Most distributions aren't tied to a specific GUI though - usually there's either different editions to download (e.g. Ubuntu's KDE and Xfce variants, named Kubuntu and Xubuntu respectively). In other distributions like Slackware, several GUIs are bundled in a huge download and you select which one you want to use during installation. You can always install a DE/WM (Desktop Environment/Window Manager) yourself after installing the distro, though it's often best to use a bundled option.
If you can't be bothered with the benchmarks, KDE seems to be the best for gaming. It's the most heavy-weight, but has an option to suspend compositing effects (and I think suspend a lot more than that) while games are running. Imagine if Windows 7 fell back to a Windows 95 style (or even DOS-style) interface behind the scenes when you fire up a game, and resumes Windows 7 interface when you leave the game. For something lightweight all the time (so faster than KDE in general and almost as fast as KDE for gaming) then Xfce and LXDE are good GUIs to use.
That said, Linux is hardly the best gaming OS anyway. As a measure of quality, personally I have very little tolerance for bugs, glitches and anything hardware-related that makes life difficult compared to Windows. In terms of the least glitchy Linux distribution, I'd vote Ubuntu (and its variants like Kubuntu). I've tried quite a few distros (at least 20, probably closer to 30) and wouldn't consider using any again other than Ubuntu/Kubuntu/Xubuntu/Lubuntu (and failing that, Debian).