Lava rocks are not required for use on a gas barbecue. Lava rocks are used to help create the traditional smoky barbecue flavor from the fats that drip onto them and then evaporate. Common grills today use vaporizer bars that are placed above the flames instead of lava rocks.
Other rocks are not safe to use with gas barbecues because they can retain internal moisture that expands when heated, causing potential explosion. Don McGlaughlin, owner of the Chicago Combustion Corporation, invented the gas grill known as the LazyMan in the early 1950s. He picked up some lava rocks from his garden and placed them in the grill when he was creating his invention.