You may purchase a firearm as a gift as long as there is no exchange of money, items, work done, sexual favors, etc. I've had to deny a firearm sale before because I overheard a dad telling his son (as he was filling out the 4473) that he would buy the gun for him, but that the son would have to work all summer to pay him back. That's a straw purchase, not a gift. As long as there is no exchange of goods or services, and is a legitimate gift with no strings attached... you are good to go.
I could be wrong, but I'm pretty sure a straw purchase has nothing to do with payment.
as long as you are not buying the gun for someone who can not legally own it, you are good.
I pretty sure I can make a deal with my daughter and agree to buy her a gun in exchange for her cutting the grass all summer.