I'd start by reading your gas meter and calculating your gas consumption per day for your last months bill and since the last meter reading.

Also look for a code indicating that the meter was estimated. The meter might also have been misread.

I have GT HVAC as well in a similar size house as yours - but we have tankless hot water. That 160 sounds high to me but its hard to tell. My monthly gas usage averaged $65 per month all last year - and that was paying 3.50 per gallon for LP. We live in the Blueridge Mountains in N VA.

We have gas fireplaces but only use them for emergency heat when we loose power and when the wood burner we have cannot keep the house warm enough is which is almost never. So almost 100% of our Lp gas usage is for cooking, dryer, and hot water.

Do you have 2 stage GT heat pumps and is the second stage gas? If so - it could be the auxiliary heat coming on. I have electric aux heater packs on the GT that I call my toasters. I wound up turning off the circuit breakers to those heater packs because when they kick in - the electric bill goes through the roof. The GT system really does not need the second stage unless it is below the design point which for me is an outside temp of Zero which we almost never see. If it ever gets that cold, i will turn those breakers on if I see the Gt not keeping up.

Let me know if you have gas aux heaters - then I will have a few more questions for you.

The other thing i would check is the hot water tank - what is the setting? If the setting is way up that could be sucking up gas but hard to believe at 160 per month. We had a cheap gas hot water heater in our rental during the build and during late spring, summer, and early fall (when we were off the gas heat) I hardly saw the needle budge at all on the tank so usage was very low just to heat water. No where near 160 per month.

Do you have a hot water recirculating pump in the house? if so - explain how it is set up.

"It's 5900 sq ft"

Wow. It's a castle if it's almost 6000 sq ft. What was your electric bill with a $160 gas bill? December was a cold month.