Free gift

The purpose of Free gift feature, which you can find by going to Online store > Customise > Theme settings > Cart, is designed enable merchants to pick a product, not linked to a collection page, such as a newley created gift product that can be purchased when a certain product is added by your customer to their cart.
This feature only works when you pick a conditional product, which means a free gift X will only be added if the conditional Y product is added to the cart first.