The gRPC website mentions protocol buffers in the first sentence, and the Wikipedia page says that protocol buffers are used for gRPC. I consider a defacto standard to basically be equivalent to a standard, even if that's not "technically correct".
However, C#/C++ Bond, C++ FlatBuffers, and Java Avro have some level of code generation support for gRPC. I'm not very familiar with Microsoft's Bond, but it appears to have generics for both the service and structs.
disclaimer: I'm part of the gRPC team.