Need help in oauth2 setup

I have simple google oauth2 setup with permissions calendar and gmail

now what I want is when I login on my app
it goes via normal process but only takes profile and all as in scope for login as usual.
after that I have one more screen where I can toggle on gmail, calendar.

whether to use it or not.

now when I enable toggle for any of it, it should ask google content then enable it.

but I am not able to get this done.

reason: users could disable permission access on app whenever they want