GitLab has a great issue tracker but you can also use an external issue tracker such as JIRA, Bugzilla or Redmine. This is something that you can turn on per GitLab project. If for example you configure JIRA it provides the following functionality:
- To reference JIRA issue PROJECT-1234 in comments, use syntax PROJECT-1234. Commit messages get turned into HTML links to the corresponding JIRA issue.