I am using Red Hat Enterprise Linux 6, it has Gnome GUI, but it is very simple and not like Ubuntu's GUI.
I want to install/upgrade my GNOME Desktop Environment to its latest release.
How can I do it?
You have two choices:
Wait for RHEL7 later this year, it will be based on versions of Fedora that contained gnome3 and systemd.
Compile everything from source.
Those are basically it. Even CentOS says they won't do it since it involves re-compiling too much original code. Not to mention it would be an enormous amount of work for marginal benefit.
I'll also add that once you get passed a certain point, you'll end up in the terminal more times than not, so the DE basically becomes the thing you launch gnome-terminal and Firefox sessions from.