set default .gitlab-ci.yml location to debian/.gitlab-ci.yml
.gitlab-ci.yml needs to be in the root of the git repo, according to the documentation. For a non-native Debian package, no modifications should be made outside of the debian/ directory. So that means that gitlab CI will not work by default for non-native packages, which most packages in Debian are.
It is possible to change the location per-project in settings/ci_cd. I think it would make sense to have the default be debian/.gitlab-ci.yml, then it can be set to .gitlab-ci.yml as needed.