It is safe to mount a microwave above a gas range, as long as there is sufficient clearance between the top of the range and the bottom of the microwave. A microwave above a gas range should be mounted a minimum of 66 inches from the ground. With most ranges, the distance should result in a distance of 13 to 16 inches between the microwave and the cooktop.
An above-the-range microwave can be mounted over gas stoves of up to five burners that have a BTU rating of under 60,000 BTUs. It is not recommended to install a microwave oven above a gas range with a BTU output over 60,000 BTUs.