Whenever a gas appliance is used, it produces small amounts of carbon monoxide. If the stove is not properly ventilated and checked out by a professional every year or so, gas stoves can leak large amounts of carbon monoxide into the air and cause nausea, headache, fainting, and death. Microwaves do not emit carbon monoxide.
Gas stoves produce heat, where microwave ovens use microwave radiation. Because the oven must operate at high temperatures, there is a risk that the oven or objects around it may catch fire without warning.
Gas stoves and other appliances also pose a bigger threat than microwaves because if a gas line becomes damaged, gas could be released into the air. Breathing the gas can cause many of the same symptoms as carbon monoxide, but gas is also highly combustible and could potentially cause a fire and even an explosion under the right conditions.