I had an issue with them regarding electric charges, not gas. I was living in another state for the summer (also a National Grid account).

Absolutely nothing was plugged in at my MA residence, yet my bill spiked high during that time period. I came home to find my upstairs neighbors (the landlord's relatives) running 5 air conditioners in their windows.

When I presented National Grid with the bills from both locations, they said I would need to hire my own electrician to investigate. They were no help at all.

I hope you have better luck.