if comfortable answering, why protobuf instead of gob?
Furthermore, Protobuf is an IDL, not a full-blown programming language. This makes it ideal for this use case, for describing APIs and data structures.
Gob-encoded data structures are described with Go. While Go is great for writing server-side business logic, it is not as well-suited as a description language for data that you need to share with non-Go systems.