If you can't start the X-server (through the startx command), then your XFree86 implementation is not set-up properly. Gnome will be available if startx works. By default, Mandrake 9.1 will start in runlevel 5, which is at the graphical level. You obviously must be starting up at runlevel 3, which is at the command prompt. So something is wrong.
Try first by logging on as root. Then try using the XFdrake utility to set your graphics up. If that doesn't do it, you need more work on the graphical subsystem.
If you don't get anywhere with XFdrake, you need to specify what type of graphics hardware you are using for us to help you set-up your graphical subsystem.