Every Confluence user needs to be authorized to access a private GitLab project to see content from this repository.
When including content from private projects, you will get the following message in place of the content:
Once the page is published, the users can authenticate with GitLab by clicking on the Authenticate yourself link.
If the user is already authenticated, all content included from repositories the user has access to in GitLab will be visible.
It may be necessary to authenticate regularly given GitLab does not provide refresh tokens.