The link to Download Package Via Mobile App is not shown when viewing a "Received a package" email, or when added separately under another hyperlink.
The reason the hyperlink is not displayed is because they are formatted in the example format below:
This format is not seen by Gmail and other mail clients as a valid URL so it does not create the hyperlink. In Apple Mail, the hyperlink is created properly and displayed.
One solution to this issue is to use MOBILE_PACKAGE_URL in the notification without hyperlinking it. This will cause Gmail to display it as plain text and the user can then copy and paste the URL into their browser.