I just put in a quick call to my favorite appliance supplier, to be sure of my facts, as I knew this answer, but wanted to confirm the reasons why before posting.....
The clearance required for a range hood over a GAS cooktop is usually 30 to 36".
Those microwave range hoods are ideal over an electric or an induction cooktop, as you can get away with an 18-20" clearance.
(a microwave installed so the bottom of it sits 30" above the cooktop is only good if you happen to be 6'6" or taller!)
As well, with those microwave combo units, they are not very deep, generally almost the same depth as your upper cabinets, and for a gas cooktop, the range hoods that vent them are generally 20"-24" deep.
I never ever do a microwave combo unit over a gas cooktop for these reasons. In addition, it is also not recommended to do a downdraft vent (the one that pops out of the counter) with a gas cooktop.
If you are switching to gas, budget for a new range hood designed for proper ventilation and clearance, and a new microwave, elsewhere in the kitchen.
Just because a photo shows the combination, does not mean it was done to code.
Q
Maybe sell the MWO?
Q