I think it was no good idea to install X first from testing, and gnome afterwards from stable.
You should imagine, that gnome doesn't know anything about Xorg, because at the time the packages were built in stable only XFree86 was part of Debian. If you had installed gnome first from stable, and after that Xorg from testing, Xorg would have complained about dependencies and solved them. I.e. upgraded gnome to whatever it had to be to be used with Xorg.
Maybe it is not completely true what I say, and should downward compatibilty be maintained at any time. --But-- your case showed that it did not work.
Why did you install Gnome from stable, and not from testing? What happens if you upgrade gnome now from testing?
I assume testing is stable enough for you, otherwise you would not have risked to install the X-server from testing.