How can I add a custom message to gift cards purchased in my store?

Shopify’s lineitem object from the order is not accessible in the Gift Card notification email which is what I’ve been trying to do. I do have access to the Gift Card object and it’s properties but those properties and the lineitem properties are two different things. When I try to access the Gift Card properties, they are empty regardless of whether the line item in the order that created had properties. That’s where I’m stuck.