GitLab has a great issue tracker but you can also use an external one such as Jira, Redmine, or Bugzilla. Issue trackers are configurable per GitLab project and allow you to do the following:
The configuration is done via a project's Services.
To enable an external issue tracker you must configure the appropriate Service. Visit the links below for details:
To save you the hassle from configuring each project's service individually, GitLab provides the ability to set Service Templates which can then be overridden in each project's settings.
Read more on Services Templates.