$ sudo apt-get remove kdm-gdmcompat 

Uninstall kdm-gdmcompat including dependent package

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

$ sudo apt-get remove --auto-remove kdm-gdmcompat 

Use Purging kdm-gdmcompat

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

$ sudo apt-get purge kdm-gdmcompat 

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 kdm-gdmcompat 

See Also,