I'm not so sure. It depends if it is a coupon, or a gift certificate. coupons usually can't make things free, but coupons also are usually for a specific item. In your situation, I think you have more of a gift certificate, so yes, if you can find a $5 game, you should be able to purchase it with said coupon/certificate.
You can go to the map select, for example, when you're starting a custom game, It'll ask you which map you want. The one's with stars by them are the DLC. Select that, and it will bring you to the Xbox Live market place where you can choose to purchase maps. If you are looking for forged (customized) maps, you can simply search online for some, or make your own.