Apart from this boring grumbling, the project is very interesting. Waiting for internals (https://marcobambini.github.io/gravity/internals/index.html) to arrive.
All serialisation frameworks I've seen map internal record types (structs, objects) to an external schema, and write directly to record fields -- breaking down that "gate". What I would like is to map those schema to something like a constructor/factory parameter list instead. (Python keyword arguments come close, but the lack of typing means there is still no schema).
Serialisation libraries should confine themselves to converting between (typed) key-value pairs and wire formats. Then there should be syntactic sugar to help the transition between that plain-old data and the real domain model.