Are you considering moving your Name Server to Amazon Lightsail? You may end up looking for what that “subdomain” means for the MX records.
As someone who volunteers for non-profits, I have the opportunity to try out Gmail from G-Suite. As of today (August 25, 2018), Google is yet to provide documentation for Amazon Lightsail DNS.
What you need to do is add your domain name to that field.
Try checking if the records are propagated using https://www.whatsmydns.net/