Commercial oven cleaners should only be used in a gas oven if the manufacturer's instructions indicate that chemicals are safe. If not, a paste of mildly abrasive baking soda and water makes a good substitute oven cleaner.
Many commercial cleansers scratch the interior of a gas oven, and metal scouring pads should never be used as they also scratch the surface of the oven. Oven racks should be removed and cleaned in warm soapy water. Scouring pads are safe for racks, and it is important to rinse and dry them before putting them back in the oven. Soaking the racks in the bathtub provides more room.