A GitLab administrator can add a service template that sets a default for each project. After a service template is enabled, it will be applied to new projects only and its details will be pre-filled on the project's Service page.
In GitLab's Admin area, navigate to Service Templates and choose the service template you wish to create.
In the image below you can see how a service template for Redmine would look like.
For each project, you will still need to configure the issue tracking
URLs by replacing
:issues_tracker_id in the above screenshot with the ID used
by your external issue tracker. Prior to GitLab v7.8, this ID was configured in
the project settings, and GitLab would automatically update the URL configured
gitlab.yml. This behavior is now deprecated and all issue tracker URLs
must be configured directly within the project's Integrations settings.