As the official resource of Auth0 states:
Performing tests against Auth0 APIs may lead to your account being rate limited, so we recommend creating mock Auth0 APIs during testing. Depending on your development environment, your test tools may also provide mock API functionality. There are also numerous API mocking tools available, such as MockServer or JSON Server, that enable you to quickly create fake APIs for testing. You can also use Postman to set up a mock server.
This means that you need to introduce mock of Auth0 API for your application under test. The idea that stands behind this statement is that you should not load-test neither Auth0 services nor Google services since you wont be able to impact their algorithm or fix anything. You should load-test your own code so you shouldn't be in prod environment for doing that.
Once you have everything set up, you load test your app like you would do in a regular case.