I remember asking myself exactly the same question some time ago. There is third party support for PostgreSQL [1] and MongoDB [2] which allows (in theory) for collaboration as the database may be hosted anywhere. However, I haven't tested this and it's far from trivial to be set up securely for most people I guess. Also, I don't know if you can configure some sort of permission management in such a setup which is a must if you don't trust your collaborators.

[1] https://gramps-project.org/wiki/index.php/PostgreSQL [2] https://gramps-project.org/wiki/index.php/MongoDB