$ sudo apt-get remove gdm3 

Uninstall gdm3 including dependent package

If you would like to remove gdm3 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove gdm3 

Use Purging gdm3

If you use with purge options to gdm3 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge gdm3 

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 gdm3 

See Also,