Hello, It would be nice to have an open source and free alternative to firebase such as mongodb.
Well mongoDB is not an alternative to firebase, as firebase is a BaaS, not a database :p. With a database like mongo DB you don’t want it to be accessible via the game directly, that would allow anyone to write anything however they want on your database.
What you actually want to do is write an API that has access to your database and exposes some endpoints to be able to interact with the database (for example, addscore endpoint could add a score to the database if you are authenticated, and a getscore endpoint would get scores even without being authenticated, etc) to validate the data, verify the autorisation to write the data and not let anyone use your database for their own projects.
You could host such an api as a REST api, and you could interact with it via the network actions, no need for an integration in GDevelop.
Have a good day