As mentioned in the documentation you lined when you’re configuring the G Suite (Google Apps) integration you need to register the client application in Google with the Web Application type. This will give you a client identifier and client secret and also allow you to continue the configuration as documented.
You need to have in mind that, in general, when going through Auth0 your client application (iOS in this case) will authenticate against Auth0 (Auth0 can then delegate to another upstream IdP, but in most cases that’s indifferent for the client application itself). Additionally, the way Auth0 will delegate and perform the authentication with G Suite is based on flows specific to web applications; hence the need to configure the client application in Google as web applications instead of iOS.
With this layer of abstraction you can then configure different client applications (iOS, SPA, etc) against Auth0 while still reusing the same G Suite configuration.