Fred, I believe that the "iron" pipe is actually steel pipe manufactured to ASTM A-53. Pipe is still manufactured in "Iron Pipe Sizes" (IPS) but the nomenclature is really archaic. About the only iron pipe you might find in a house is cast iron used for DWV applications. Most building codes in the country tend to follow the American Gas Association (AGA) recommendations for allowable products.
When you pipe a water heater or many other gas appliances, put a tee in the vertical line with a horizontal nipple to run to the gas valve and another nipple and cap on the bottom of the tee. This gives a space for dirt or scale to collect so that the dirt does not get in your gas valve or clog your burner. And don't forget to check all your joints with soapy water. Bubbles will indicate a leak.