When someone in the USA gives you something as a gift, or you are sending someone some item as a gift, you do mark "gift" on the customs declaration. In all other cases, you must select "merchandise."
A gift is when someone sends something to someone free. You most likely will do not pay Import duty, Tax or any fee, if the value of the item is below certain thresholds. If you buy something and send it to another person usually does not satisfy the custom’s definition of a gift.