Appliance repair, many work on gas appliances,
I would call and see what they do, yes there is a a fair amount of cross over, when it comes to appliances and heating and gas appliances,
I would just start calling and possibly start with your gas supplier, and if I would call a plumber ask before having him come, yes he may change out thermo couples in a
hot water heater and an occasional valve but his main job would most likely be certified in the running of the gas piping, not the repair of the appliance,