GDM is a system service that is responsible for providing graphical
logins and managing local and remote display.

A note on PAM:
The GDM source includes PAM configs for linux distros like redhat, lfs,
and arch. However, none of these pre-designed configs are suitable for 
Slackware's PAM configuration. Therefore I used 'sddm' as a reference
and wrote configs for 'gdm' that work properly. I didn't include 
'gdm-fingerprint' or 'gdm-smartcard', since I don't have the hardware 
needed to properly test functionality. 

Additional Notes:
- Slackware comes with the 'gdm' group and user already configured, so 
nothing needs to be done there.

- The blocaled dependency is used to provide custom keymap support for
  'gdm'.