Adding solar PV to your structure typically will not lower your natural gas bill, just your electric bill. But one does have the opportunity while sizing the solar PV system to make changes to how you use energy that will lower the natural gas bill. You have the option to swap your gas hot water heater with a tankless electric hot water heater. By switching to electric hot water one would want to then upsize the solar PV system slightly so your potable hot water is produced by free clean solar electricity.
Another way to lower you gas bill with solar photovoltaic is if you have cold room(s) in your home that causes you to turn up the thermostat for your gas furnace just to get those rooms comfortable while overheating the rest of the home, then here is the better solution. Turn the entire home’s thermostat back down to a reasonable temperature setting that keeps the bulk of your home comfortable and place portable electric heaters in those rooms that are chilly. Your furnace will work less and thus you will buy less natural gas and the electricity used to make the cold rooms comfortable is coming from a free, unlimited and clean source of power temperature, the sun.
One could also change from a gas clothes dryer to an electric dryer, gas cooktop to an electric induction cooktop, and you get the idea. Why buy gas if you can make energy from the sun to do the same functions in your home.