Daniel asked a pertinent question. There is very little information to go on here.
Did you ever get the nvidia driver working, as in: did you see the nVidia splash screen upon loading X?
You would not use "startx" and GDM together. You either log in via the graphical login manager, or to command line and issue command "startx". Since you mention GDM, I guess you do get a graphical login screen? If so, what do you see after entering username and password?
Use Ctrl-Alt-Backspace to kill X.