Unlike MockMvc which is a server-side testing API, the WebTestClient is an actual client and has the same features as WebClient including deserialization. It's easy to convert the output to a specific Object, a List of Object's, a Map of Object's, or a stream/Flux of Object's. This makes the JSON content and JsonPath options less crucial than they are with MockMvc.
That said the plan is to support JsonPath still, so I will turn this into a request to add JSON content and JsonPath support.
I don't know what you mean by "standard" but for the time being you can get the response body as a String:
EntityExchangeResult<String> result = client.get().uri("/path")
// Use JsonExpectationsHelper directly on result.getResponseBody()