Putting ANYTHING against your paint is going to scratch it to some degree. Even assuming a perfectly clean gas tank and tank bag, which is never going to happen but anyways, as the tank bag shifts around slightly from the bike movements it will do minor damage to the paint. Add on dust, rain, all the other junk in the air into the mix, and that tank bag is going to leave some history on the tank. Best option is 3M film like tank slapper, or just live with the consequences and try to keep things as clean as possible