gRPC generates rpc server and client stubs based off a protocol buffer definition. Saying it's a replacement for REST makes little sense since it's possible to define a REST API within it. It's also possible to write a totally not RESTful API in a modern http api framework.

In fact that's what Google's API design guide does. Encourages RESTful API design then describes how to implement them using proto and gRPC.

The more interesting tradeoffs are proto vs json or other and how this restricts message patterns to request/response (rather than pub/sub or push/pull)