One would expect that considered the complexity of gdm there would be a way to have per user X11 options. So no, there isn't one. At least not after the great rewrite after V2.22.
However good old Unix paradigms can help us (this is all under Debian, other Unices will allow a similar trick):
Add something similar like this:
We check whether gdm is starting us and using the parameter “–username le_gamer”. If it is, then we're using a different config file for X.
Of course you'll need to adapt all this, unless you are using Debian or Ubuntu. You need to adapt the path to the X Server, to the config files etc.
Why did I do that? The problem is that Intel's X server in Ubuntu Lucid Lynx 10.04 is really unstable with DRI on a Intel GM965/GL960 graphics card.
So unless I'm playing 3D games which are a lot faster with DRI, I don't want to enable DRI (more over, “NoDRI” is pulling less power out of my battery). So my normal config contains this:
So now, the only thing you need to do is replace your “normal” X Server with the new “adaptable” one: