Integrating TeamCity with GitLab Issues
TeamCity integration with the GitLab issue tracker can be set up separately or as a part of TeamCity integration with the GitLab source code hosting service.
Configure connection with GitLab as described here. When setting up integration with GitLab issues (see general information here), in addition to the repository URL and other general settings, you need to configure authentication and specify the issue ID pattern.
TeamCity allows you to select whether you want to connect to GitLab anonymously (for public projects) or to be authenticated via a personal access token (PAT).