Probably...
You should look for something along the lines of a package manager that allows you to select alot of RPMs, and then insert a disc and it runs through the install while you go have a cup of joe
If you don't find something like that (I don't know RH very well, but Mandy and SuSE both had this) then you will need to find the rpm's relating to GNOME to install. You can probably figure out most of them either from unsatisfied dependencies, or maybe from
www.gnome.org
This would be the way I'd end up doing it anyway if I didn't wanna lose my custom setup.
Some people have had success from simply putting in your distro's install CD, getting to the section to choose packages, and choose the GNOME package. It updates their setup with the files, not redoing the whole thing, and everything works fine, but I would back things up if this is what you choose to do.
Cool