Yes it’s possible to use the gRPC API to support server-to-server communications. You’ll need to use the service definition defined here https://github.com/heroiclabs/nakama/blob/master/apigrpc/apigrpc.proto#L535 and run it through the proto toolchain. This will generate the appropriate code you can then use to connect and send/receive data as required.
I’d like to try and understand your use case for communicating between servers via gRPC. Can you give me a run down of what it is you’re trying to achieve? That way I can try and assist you in finding the optimal solution.