1) contact the local Call Locate Center to get proof that a locate of utilities was done - get proof in writing - they keep a record of all calls - who from, what for, where, etc - and the utilities indicate to them by compuer when locate was done.
2) get a copy of the paperwork between the contractor and the gas company - that should state, with contractor's signature, whether it was a bad locate, or if he dug with a locate and was not where the locate showed the line, or dug where the line was indicated and hit it. Also read if the form says who is deemed responsible for the line break.
3) If a locate was done and either the contractor was deemed to be not at fault in the paperwork, or he was deemed at fault and signed to acknowledge it, you may be in the clear.
4) Unfortunately, generally ANY utility damage (other than natural causes) done on your property or by contractors working on your property (excepting caused by others using an easement likek a road easement where a car hitting a power pole or hydrant or gas meter is charged to them, not you) is generally deemed to be the responsibility of the property owner unless a proper locate was done and adhered to and a buried line was hit anyway. Then it is up to the homeowner to recoup the cost from the responsible party.
5) Certainly you can fight it - first with the documentation above (assuming it is in your favor), and documentation that the utility guy said you would not be liable - get the crew's names from the company, and find out the name of the one who talked to you and demand a face to face meeting with him and a supervisor to get him to admit he said you would not be held liable.
6) Beyond that, you could file a formal written protest to the company, challenging it all the way up to the board of directors of the company, and even to the State Regulatory Commission. While being appealed, they cannot try to collect on it, but if you ultimately lose you can be charged the fee plus interest. After the utility route, you could go to court - Small Claims probably in your case, as an attorney would cost at least as much as you are getting back.
7) Bear in mind tht these days many corporations are in the mode of bill the customer for everything - if the customer pays that cuts overhead costs - if they fight, the company may back down immediately, so talk to a customer service SUPERVISOR first - it may go away at that point, BE sure to get in WRITING that they are revoking the charge and that you are NOT responsible for it, so it doews not come back later.
8) Of course, 20-20 hindsight, but was your contractor bonded and insured ? Bonding might cover this - sort of a gray area but damaging ancillarly constructed property in the course of the work is generally covered in construction bonds - but the contractor's insurance certainly should. And of course another 20-20 hindsight - your contract should have made clear whose responsibility it was to get locates - it should always contractually be the Contractor's responsibility since he is the one controlling schedule and actually doing the digging, but you should make sure it was done before excavation starts.