Did you arrange a last reading / disconnection of gas supply, and a final account ?
If so, did you take a final reading / photo of the meter yourself as a bit of insurance ?
I would always take such precautions, and there are many ways this could blow up on you.
You were renting ?
Are the bills showing a supply charge only, or if gas is being used an amount that shows someone is living in the place now ?
Hope you can get this sorted out, but if you don't arrange the service to be finalised on the date you move, you could be up for the bill until it is done.