Need a help with setting a OAuth consent screen in google

This is first time developing mobile app and using google sign in.
I am trying to implement “Sign in Gmail” function in my app and my app is in very beginning.

So, to create client id for both ios and android, I need to set OAuth consent screen and only available options between internal and external is external.

It requests my app’s information for verification and it takes 4-6 week.
But I am developing app and not even ready to publish…

So how can I test google sign in while developing app? or “google sign in” should be implemented later when app is ready?