Drum roll!
The above command will only work as root, and it will ask GDM to load up. With any luck, it should give you....
No luck? Go back to your IRC console, and ask somebody for help - there should be a #linux channel on every IRC network with somebody to ask for XFree86 help. Or you could try our own #HEXUS.net on irc.quakenet.org. Either way, somebody is out there on IRC to help fix it.
For the rest of you (and I hope that's most of you), welcome to your new graphical Debian GNU/Linux system! Log in, and get cracking on exploring the options. GDM will come back every time you start Debian from now on.
Look around, experiment, try out the applications available to you! If you want more programs, you can search for things inside a terminal window (you can open a terminal inside Gnome) by typing "apt-cache search somethingtosearchfor" - then apt-get install one of the results to install it. If you need help on a command, look in the manual - "man commandname" in a terminal, using the "q" key to quit. There are graphical front-ends to the apt tools which you can install, such as Synaptic (apt-get nstall synaptic) - though in my experience the searching in apt-cache is far more effective. Equally, there are text-based front ends such as aptitude or tasksel. Try the options available to you, and find your sweet-spot.
Once again, if you need help, ask in IRC! You can also find some information on The Linux Documentation Project. Play about, and above all, have fun. For a big list of Windows applications and their rough Linux equivalents, try here.
Part 3 of this guide will cover some tweaking to turn your Debian system into a lean mean gaming machine - with tips on installing 3D-accelerated graphics drivers, motherboard temperature sensors, DVD and video players, improved sound drivers, and games - both Linux-native and Windows-only through a little thing called WINE. See you then!