Mocking methods in test

I have a class i want to test:

type ApiGateway struct {
    username  string
    password  string
    scsClient *scsClient.APIDocumentation
    auth      Auth
}

type Auth struct {
    token   string
    validTo int32
}

func New(host string, scheme string, username string, password string) *ApiGateway {
    var config = scsClient.TransportConfig{host, "/", []string{scheme}}
    var client = scsClient.NewHTTPClientWithConfig(strfmt.Default, &config)

    return &ApiGateway{username, password, client, Auth{}}
}

func (s *ApiGateway) isTokenValid() bool { ... }

This isTokenValid method is called from every other method inside the class in order to verify and/or update the API token. In tests I want to mock this method, so it always returns true for example.

How do I archieve that?