Let me paraphrase your question:
"Hey, if I don't mention this to anyone, when the gas company
eventually discovers it, will I get billed retroactively?"
The answer is: No one here can say.
What's the matter, you feeling guilty or something? Why on earth would
you come here to ask that question.... you know what the right thing to
do is: Call the gas company. Now. You cheap ass SOB.
I would like to thank you, and others like you, for making the price I
pay for my gas higher. It's so nice of you. I bet you like to shoplift
too, don't you?
While people here may answer your supposed question differently - none
of them can supply you with a sense of right vs. wrong. You were
supposed to learn that lesson years ago; evidently your parents are
bigger scumbags then you.
Your lesson is complete now, scumbag junior. Run along.