@Lars Gausling - Welcome to Microsoft Q&A and thanks for reaching out to us.
Gantt chart plugin is no longer being maintained and the author recommends using the State timeline panel anyway. Managed Grafana doesn't provide support on how to use standard Grafana features. For these, you should refer to Grafana's documentation: State timeline | Grafana documentation
Hope this helps. and please feel free to reach out if you have any further questions.
If the above response was helpful, please feel free to "Accept as Answer" and click "Yes" so it can be beneficial to the community.