Well, you have said it yourself. gRPC is a framework that uses RPC to communicate. RPC is not Protobuf but instead Protobuf can use RPC and gRPC is actually Protobuf over RPC.
You don't need to use Protobuf to create RPC services within your app. This is a good idea if you are doing libraries/apps from small to medium size. Also you don't need to learn the syntax of Protobuf to create your own services.
But, Protobuf is much faster than REST. It is a much more convenient way to communicate with the downside of the learning curve of the Protobuf syntax. Also, you can use Protobuf to generate the codebase in more languages than simply Go. So if you have some kind of service in Java, you can use Protobuf to generate RPC calls between them easily while if you use the net/rpc package you'll have to implement them twice (once in Go and once in Java)
In general, I will use Protobuf to nearly all. This gives you confidence to use it at more large scale or complex projects.