Backtrack Linux is not configured by default to load a display manager, so there is more work to be done than just installing GDM. Here's a step-by-step of one way to install and enable GDM in Backtrack 5 R1.
First, thanks to @DavidVermette for the YouTube link. This video covers all the steps, albeit in a different order and with little to no explanation: http://www.youtube.com/watch?v=9uMqsVFVO58
NOTE: Some of the commands or procedures below may require elevation, though I'm not sure which. In a default install of Backtrack 5, you're running as root anyway so this should not be an issue unless you've set yourself up to run as a limited user. In that case, (and since you're running Backtrack in the first place) I trust you know how to troubleshoot "I need to do this as root" issues yourself.
Firstly, of course, you need to install GDM. This can be done with the following command:
apt-get install gdm
Next, you need to configure the system to load GDM at startup. This can be done by editing
/etc/rc.local to include the following line:
Remember to leave
exit 0 as the last line in
/etc/rc.local and save it.
Last, you will probably want (as I did, in the question posted here) to load the X Windows interface automatically after login. This can be done by adding the following lines to
.bash_profile in the home directories of any users for which you want it applied.
In the case of a default Backtrack install where the only user is root, the only file you need to worry about is
Optionally, the video linked above also walks you through setting up an extra user account. This is not necessary for GDM to work, or for the system to auto-start the desktop - I imagine it's included merely for aesthetics or some personal preference.
After all of the above, reboot your system and you should see the settings have been applied. GDM will load to prompt you for your credentials and give you some other options to pick for your desktop environment. After successful authentication, your chosen desktop environment should load.