The Ontario Building Code (OBC) stipulates a minimum vertical clearance of 750mm (30 inches) from a cooktop to combustible materials such as framing, finishes and cabinetry, unless protected by a non-combustible surface (eg: sheet metal, a metal range hood, etc.)
The clearance between a microwave oven and a range top will vary based on what kind of microwave you're planning to use. If you want to install an over-the-range (OTR) microwave, you will need much less space between the stove and microwave than you would for a countertop microwave as these models are designed to withstand higher external temperatures.
OTR microwaves, which are designed to tolerate less clearance, should be mounted according to their manufacturer's instructions.
Keep in mind that you will potentially be reaching over a hot surface to use an OTR microwave.
Note also that a lower position can crowd the workspace and make it difficult to lift lids off large pots. Depending on your height, a higher position can increase the risk of spilling hot food on yourself.
Large commercial-style gas ranges with total burner outputs of more than 50,000 Btu should really have a range hood vented to the exterior.
Here’s a few well-written articles of interest regarding range hoods, OTR microwaves and other applianaces.
Can You Put a Microwave Above a Gas Range?
Kitchen Range Hood Vent Installation
Clearance requirements for a variety of appliances
Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario since 1994.