What the difference between JSON RPC with HTTP2 vs grpc?
As you pointed out, gRPC and Protobuf are often conflated. While, in the vast majority of cases, gRPC will be using protobuf as an IDL and HTTP/2 as the transport, this is not always the case.
So then, what value does gRPC provide on its own? For starters, it provides battle-tested implementations of each of those transports, along with first class support for the protobuf IDL. Integrating these things is not trivial. gRPC packages all of them into one nice pluggable box so you don't have to do the legwork.
It also provides you with functionality that HTTP/2 on its own does not. Pluggable authorization/authentication, distributed tracing instrumentation, debugging utilities, look-aside load balancing (including upcoming support for the xDS protocol), and more are provided.