When Delivery Gateway creates a shipment and marks the Shopify order as fulfilled, Shopify can send its shipping confirmation email to your customer with the tracking information.
This is controlled by a setting in the Delivery Gateway Shopify app under App settings. It is turned on by default.
If your customers don't receive the email:
Check that the setting is turned on in App settings.
In the Shopify admin, check that the Send customer notification is enabled under Customer notification.
If both are on and the email still isn't sent, contact us in the chat with your store URL and an order number.