Java: How to mock external api responses

I have a Spring API Gateway implementation and I want to test its ability to make a call to an authentication endpoint and resolve if a given request is allowed or not (e.g. JWT is valid).

I've seen solutions like Wiremock that allow me to stub responses to requests made against localhost. But since I'm using a service discovery mechanism I need to mock a response from an 'external' host.

I've seen this implemented in other languages, but how do I do that using Java?