Hi!
The GitLab Issues are linked as workitem to the testcase.
The workitem API is not officially supported/documented yet, but I can share it here so that you can already use it. In the future, we’d want to offer an easier way to create/link issues.
To read the issues back on the web api, specify the mapped entity workitem as described in this answer.
To create workitems and then link the workitem to a testcase, you can refer to this answer in the same thread. It describes the behavior for Jira, but the general workflow is the same for all integrations.
You can also check the API calls that are made when you link a GitLab issues via the GUI in the DevTool’s Network tab.
I hope that helps you to get started 
Best regards,
Hanna