By design, the purchase of a Gift Certificates cannot use Discount Coupons ...

You could make a regular Product, that does not use the products_module starting with:
GIFT

and have the Discount Coupon made for this one Product with a 1 use per customer and 1 use total ...

Then set the Restriction on the Discount Coupon to only allow this one Product ...

Once Purchased, you can then mail the Customer a Gift Certificate from the Admin for the amount that you wanted them to get it for ...

Then, when they redeem the Gift Certificate they will have the full amount that you intended ...