Call your gas company and let them know the condition of your main shutoff valve. I assume this meter is in an outdoor location.
First, as others have said, do NOT loosen that nut on the backside of the valve. It should never be loosened unless that valve is removed from service for maintenance.
Second, all gas company valves I've seen prior to a meter have a locking hasp. The gas company is required to close a valve to a residence and lock it closed during certain modifications and prior to energizing the gas line after new construction and only after they inspect the gas lines and appliances.
See the two pictures below.
I would like to see the other side of your valve. Could the wrench tab and the hasps be broken off of your valve and have been painted over multiple times?
And you are absolutely correct, that that main valve should be able to be operated by you in the case of an emergency. If your gas clothes dryer in your basement stops working and your wife pulls it out and it falls over breaking the gas connection, and she runs upstairs, you have three main options.
1. Run down the stairs and close the gas valve on the line running to the dryer.
2. Call the gas company/fire department and have them come out and close the main shutoff valve to your house.
3. Go outside and close the main shutoff valve yourself.
You could also stop paying your gas bill and I bet the gas company would come out and close that valve and lock it somehow, which as I see it would require replacing that valve.