I think you're getting confused somewhere, as Grub has nothing to do with the choice of Gnome/KDE - they're on completely separate screens. Grub controls how the machine starts up; Gnome/KDE control your login session.
It sounds like you're at the login prompt screen. There isn't a generic login name you can use, and I doubt if it will let you log in using the root user. You should have created a username when it prompted you during the installation!
Your only option now is to log into text mode and create a user manually:
Hit ctrl-alt-F1. Log in as root using the username root and the password you chose during installation. Then do this:
useradd -m -p mypassword myusername
Change mypassword and myusername to be the username and password for the new account you are creating. Hit ctrl-alt-F7 and log in with this account...you should be up and running.
For what it's worth, you'd probably be better off using a more recent distro than RH7.3, which is very old. RedHat is now a paid-for distro but Fedora Core 5 is supported by RedHat and you should find it's very similar.