If you are talking about from a ttyx, then you would type
# startx /usr/bin/gnome-session
That launches GNOME and all the fixins'
AFAIK, you can't launch it from an already launched X in that same X, however, from a terminal typing
$ startx /usr/bin/gnome-session -- :1
Will launch GNOME into another X (located in Ctrl+Alt+F8). You can also start this from a VT (Ctrl+Alt+Fx).