First of all, Gnome 2.2 is pretty old. You probably mean 2.20, which is pretty much newer than 2.2.
Then again, Gnome 2.20 is pretty new and doesn't necessarily be in the Fedora 7 reposities yet. It's possible it never will be, that they put it in Fedora 8 instead; or, it can be that if you wait a few weeks, somebody has finally compiled and packaged it. It depends. Did you check if the reposity has Gnome 2.20 files or not? Also I'm fairly sure "yum update gnome*" won't do much, because Yum doesn't support wildcards if I'm right..I think the way to go would be something like this:
yum groupinstall gnome
However since you've got Gnome (ver. 2.18) already installed, there's no need to go there; yum update and yum upgrade (or even dist-upgrade) should do the trick. You could just ask Yum to upgrade a single Gnome package, and it probably asked you to upgrade them all, if they were there.