There is tooling built to easily do grpc requests from the command line[0] or even a GUI if that's your thing[1].
The tooling isn't quite as ubiquitous as cURL, but if anything I would think that its even easier because you have introspection into the contract with a .proto file.
Exactly. JSON is benefitting from the fact that it can ride on top of existing non-dedicated tooling in an ad hoc way. Protobuf/gRPC needs dedicated tooling designed to replace what JSON can do with cURL. But that tooling has the potential to be much better than what can be accomplished with JSON, because it can actually type-check your input (e.g. catching typo'd field names) and maybe even let you explore the schema interactively. E.g. imagine having bash tab completion for these APIs -- easy if you have the schema.
The tooling isn't quite as ubiquitous as cURL, but if anything I would think that its even easier because you have introspection into the contract with a .proto file.
[0] https://github.com/fullstorydev/grpcurl [1] https://github.com/uw-labs/bloomrpc