No configuration needed.
By default no configuration is required. Each user will have to authenticate with OAuth to GitLab to access the content. Therefore, each Confluence user also needs a GitLab account to see the private projects.
Alternatively, shared authentication can be used. All Confluence users will share the same authentication to GitLab and therefore won’t need a GitLab account. Confluence users will see all content the authentication token has access to.
On GitLab.com, navigate to User-Settings → Access Token for user scoped tokens. Or project settings → Access Token for project scoped tokens.
Create an Access-Token:
Note the expiration date. Be aware to update the token if it expires or remove the expiration date.
On Confluence, navigate to the Settings → Include GitLab
Switch the authentication method and save the token: