If its free of cost and the person downloading the same is not related to you. Then No GST. If the person is related, then we have determine the value of Goods /Services as per Schedule 1,
As you will be getting Income by way of Advertisements, Then GST is required to be paid on that Amount. If the advertisement is in print media than 5%, if its digital Media then 18%..
Regarding the form you are required to file is GSTR 1, GSTR 3B for payment and Annual Return 9 and 9C