I would recommend contacting Shopify Support for further assistance. They can review your domain settings and escalate the issue to our Technical merchant support if they are unable to spot the issue. You can do this by logging into your store through the help center, searching your question/issue, then selecting "continue" at the bottom of the page that displays the search results.
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog
This has been a huge problem for over a year. Shopify forwarding to gmail is simply broken and so many customers have had their communications to our business completely dropped. I had a lot of back and forth with tech support over it in the past. We set up an account with Outlook and get most of the messages. But some still just never arrive. It is a huge problem and I imagine many people never notice unless their customer's find a different way to contact them and let them know.