Can I Remove GUI From Debian?

Debian uses tasksel for installing software for a specific system. The command gives you some information:

> tasksel --list-tasks
i desktop       Graphical desktop environment
u web-server    Web server
u print-server  Print server
u dns-server    DNS server
u file-server   File server
u mail-server   Mail server
u database-server       SQL database
u ssh-server    SSH server
u laptop        Laptop
u manual        manual package selection

The command above lists all tasks known to tasksel. The line desktop should print an i in front. If that is the case you can have a look at all packages which this task usually installs:

> tasksel --task-packages desktop
twm
eject
openoffice.org
xserver-xorg-video-all
cups-client
…

On my system the command outputs 36 packages. You can uninstall them with the following command:

> apt-get purge $(tasksel --task-packages desktop)

This takes the list of packages (output of tasksel) and feeds it into the purge command of apt-get. Now apt-get tells you what it wants to uninstall from the system. If you confirm it everything will be purged from your system.

It means Debian changed the behaviour of this by now using a single "meta" package called task-desktop which pulls in the other packages via dependencies and recommends. So it's true what tasksel tells you, it just installs that single package, but if you look at the details of that, like so:

apt-cache show task-desktop

you will see the other (actual) packages this pulls in in the lines starting "Depends:" and "Recommends:". To remove them try "apt-get remove task-desktop" or list the package names individually.