byzantium: Add gdm3
To enable the automatic build of gdm3 after updating the Git tree (and repo settings) based on this MR
$ GITLAB_TOKEN=... ./check-pureos-pkg-repo -i 399
Summary for https://source.puri.sm/pureos/packages/gdm.git
==========================================================
OK: 'pureos/latest' branch present
OK: Default branch is 'pureos/latest'
OK: Branch 'pureos/latest' is protected
OK: Branch 'pristine-tar' branch present
OK: CI enabled
OK: Snippets disabled
OK: Wiki disabled
OK: Service desk disabled
OK: Merge method is fast-forward