How to configure NGINX for full proxy of the other domain?

Hello friends,

the challenge trail. There is a (sub)domain mail.mydomain.com and VPS, you Need to make when opening the address mail.mydomain.com the page that opens gmail.com. While all worked well, including JavaScript, sent mail, etc. But in this case, any URL started with mail.mydomain.com without going to gmail.com. Options web proxy or ssh channel does not offer.

thank you, Vadim