There's a curl for RPCs internally. It helps too that RPC servers are self describing, so you can actually list the services and methods exposed by a server. I'd say it's much simpler than curl, although again that's in large part because there's a lot of shared infra and understanding, and starting from scratch on that would be hard.

Server reflection exists (https://grpc.io/docs/guides/reflection/), but you don't really need to whip out curl when you have the RPC service's definition. It tells you everything you need to know about what to send and what you will receive, so you can just start writing type-safe code.