Hello AccountixCA,
Thanks for sharing your insight about what might be happening with QuickBooks Online's Receipt forwarding when using Gmail and Gsuite. The requirement for that code could be part of what's causing the hang up with the process.
Since you've mentioned that Receipt Bank handles this with using a unique email address, the new receipt forwarding option for QuickBooks Online may work for this situation. Recently, we updated the process for receipt forwarding to allow users to create a custom email as a part of this process. Take a look at it here: Email receipts and bills to QuickBooks Online
Should that still not do the trick, I recommend sharing your feedback about how this feature works, which you can do right from the Receipts page in the Banking menu by clicking Give feedback. This helps us as we continue to develop the feature and QuickBooks as a whole to meet the needs of business owners and bookkeepers.
I hope that helps.
