So this solution works but there is a limitation with this approach. The Odoo setup guide recommends this for Odoo Online and Odoo.sh, but I am thinking it may be better to use a separate incoming mailboxes for the catchall alias for the reason below.

If you send an email directly to catchall, the response telling you not to email it directly will goto to the gmail account that is forwarding rather than back to the original sender. This seems like either an Odoo issue or perhaps a gmail limitation. I'm not sure which since I don't know how to inspect the original headers of the email Odoo receives.

1. Access Gmail Settings: Log in to your Gmail account and click on the gear icon in the upper-right corner of the Gmail interface. From the drop-down menu, select "Settings."

2. Navigate to Forwarding and POP/IMAP: In the Settings menu, click on the "Forwarding and POP/IMAP" tab.

3. Add Forwarding Address: Scroll down to the "Forwarding" section and click on the "Add a forwarding address" button. A popup window will appear.

4. Enter Catchall Address: In the popup window, enter the catchall forwarding address that you want to confirm. Make sure to input the complete email address correctly.

5. Verify Forwarding Address: Click on the "Next" button, and Gmail will send a verification email to the catchall forwarding address.

6. Confirm Verification: Access the email account associated with the catchall forwarding address and locate the verification email from Gmail. Open the email and follow the instructions provided to confirm the forwarding address.

7. Complete Confirmation: Once the catchall forwarding address is confirmed, return to the Gmail settings page. Click on the "Refresh" button next to the forwarding address you added to confirm its verification.

By following these steps, you can confirm the catchall forwarding address when setting up Gmail as an external incoming mail server.