Gob is Go-specific. Our mission is to make schema-driven APIs easy, regardless of what language you use. Protobuf already has official support for nearly a dozen languages, and unofficial support for many more. Protobuf also has a compiler with a plugin model, which facilitates supporting even more in the future.

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.