You could do this by adding a function for the post templates to fetch user_id and get the email and then make a custom merge tag that then can be embedded in the page where the form is loaded.

If that isn’t viable you can pass the post ID where the form was used, create a function to use one of gravity forms event functions to run similar logic and send a custom email notice that way.

Gravity forms has documentation on creating custom notifications already just a matter of having the Post ID as a value in the form entry to then use.

I would explore gravity flow since it also gives some extended capabilities that could almost take care of this for you.