I know it may be a lot to ask and no idea how much this would be appreciated, but how about integrating GanttProject into Nextcloud? Why? If you wanna be sure, your data is private and safe, don’t trust a public server…
…or is this input here at the wrong place?
What would you expect from such integration?
integration in the web interface (ideally without local installation, like Nextcloud Office or others), collaborative and/or simultaneous work on a project, possible definition of access rights for users (read/write but also assign topics to work on).
I know you offer a cloud (so it may be not such a big step), but many people/orgs/companies would not want to trust a public cloud with sensitive data like a project plan.
After all, Nextcloud doesn’t offer such solution yet and I feel, your project would fit very well with Nextcloud.