This was advertised in their flyer. If you spend $40 on gift cards you will get a $5 off coupon, now through Dec 24th.
You can redeem upto three $5 coupons per transaction. And coupons are valid from Dec 26th to Jan 31st. I don't think you need to spend any minimum amount to use these coupons, it was not mentioned in the flyer.

These coupons are valid any Old Navy store in Canada and US, but cannot be used online.