Is it possible to view the Gantt Diagram for Issue List?
Currently not, and I don’t think it’s on the list right now, but feel free to open an issue. I think this could be implemented using the api, but probably as an external service.
It has been asked on github (https://github.com/go-gitea/gitea/issues/5606) but the maintaners think it is out of scope from gitea’s scope so it should be implemented as separate project.
You also could use track time?