To create order rules, you need the Order Rules module, if you would like to purchase this, please raise a new case online and reference the title of this article.
The PopulateGiftMessages Order Rule action will populate the GiftMessages field on the order with the details from the param within the order rule. For the Param field enter the gift message you want to show on the order.
Example - Thanking customer for their purchase
You want all orders to have a thank you message.
Action: PopulateGiftMessages
Param: Thank you for your purchase!
OrderRuleCondition: (Optional)
Then, any order that imports will have that gift message populated on the order.