Most modern over-the-range microwaves state that it is fine to mount a microwave over a gas range. Be sure to allow as much space as possible due to how much heat this stove generates. Each individual microwave will have its own specifications on the clearance needed from the range top to the bottom of the microwave, but 16 inches is generally the most common. I would recommend more than 16 inches due to how hot it gets. More space is better as long as you can still comfortably reach the microwave. You also need to make sure that you have enough room for your tallest pots to fit on the back burners and anything less than the 16 inches usually makes it hard to use the back burner of stoves with tall pots.
Some states have their own code for space needed over a gas range and can be as much as 30 inches which would make it impossible to actually reach the microwave. Since microwave vents aren't very powerful, many people prefer actual vents (that vent outside) with gas ranges to cut down on harmful carbon monoxide emissions.