Yeah you’d use something like MockRestGateway, there’s not one for the complete IServiceClient API. I generally don’t mock Service Clients for my own Services myself. Although I’m more likely to mock 3rd Party API’s since they can be slow/unreliable. MockRestGateway was actually built for Stripe Gateway.
Although you could also use the ResultsFilter/ResultsFilterResponse delegates, they were built to support a client caching strategy, but stubbing/caching serves a similar role and this would let you use a dictionary collection for returning predictable responses which has the added benefit that when you want to test against the real service you can just comment out the ResultsFilter delegates.