First off, welcome to ECN!!
Actually, I'd try to state the case in the oposite direction; i.e. the gas cooktop will only pull about 1 amp or so for the light(s) and ignitor.....mention the combination range/microwave and/or microwave/exhaust fan units as possible precedents. Hardly a serious load factor for the microwave circuit.
But sometimes (actually many times) the Code turns a blind eye to reality and you may be stuck rewiring or adding a recept to the general appliance circuit in the kitchen.
Before someone I know quite well, comes on to blast me about the Code remark
I must say that the intention
of the Code in this case is a very good one. It would be quite easy to overload the gen. purp. circuit with the microwave and just one other appliance.
Usually a polite conversation with the inspector will allow this to pass. But be ready to quote Code references that will help your case.