If you setup a discount code i.e. get 10% discount on all products in store and someone purchase a $100 Gift Certificate, then NopCommerce allows the discount on the Gift Certificate (buying $100 for $90). If someone then uses that gift certificate to purchase something in the store while the promo is still in effect, they will be able to buy $100 with the Gift Certificate they paid $90 for and get another 10% discount on top of that!
I haven't had a chance to check it yet but I also assume that reward points will be awarder for the Gift Certificate as well as for the product bought with the gift certificate meaning double rewards are applied.