I am working on Google Action for Google Assistant. Our custom Action relies on user's phone number to get additional information from external server.
According to the official guide you can use Account linking with Google Sign-In to get some user information from Google ID token:
I know that you can fill out optional contact fields in Google account like address and phone number.
Does anyone know if Google ID token contains this optional information or no?
The answer depends.
Some basic personal metadata is available through Google Sign-In. With regards to the other fields:
Address
This information is not easily accessible. You can use an in-dialog permission to access the current location, but not the home address necessarily.
If your Action uses physical transactions, then you can get their address in order to ship items. But you shouldn't use this as a way to get the address if you're not shipping items, and such an attempt would be detected during review.
Phone Number
No, this is not something you can readily access. For general things like notifications, Google Sign-In gives you an email address. You also shouldn't ask it in your Action. The review team won't be happy with that, and the voice transcription doesn't work very well for long numerical sequences.
Gender
No, this is not something that is part of the Google Sign-In, and there's no helpers. With regards to asking it yourself, I don't think the review team would have a problem, although you may want to consider whether it's necessary for a good voice design.