The minute quantity if zinc presented by gas checks isn't going to contaminate anything. Zinc is not kryptonite. A small amount of zinc is soluble in lead alloy and serves as a hardening agent. As mentioned in previous posts, zinc washers (which weigh a lot more than a gas check) were widely used in the 50s and 60s in some cast bullet designs, and I've never read anything to indicate they caused any problem with contaminating range lead. If you want to worry about something, worry about range lead contamination caused by the growing use of powder coated bullets.