$ sudo apt-get remove gdm 

Uninstall gdm including dependent package

If you would like to remove gdm and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove gdm 

Use Purging gdm

If you use with purge options to gdm package all the configuration and dependent packages will be removed.

$ sudo apt-get purge gdm 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove gdm 

See Also,