Are you sure your gas bill isn’t for two months or something?

$600 bill at $1.24 a therm comes to 483 therms in a month, which comes to 48.3 million BTUs. A house your size would have 110k BTU furnace, even at 120k BTU that kind of gas load is having that furnace or boiler running flat out 12 hours a day all month long, plus like 2 hours of stove/dryer a day. The weather hasn’t been nearly that cold for that to be happening.

I’d say something is wrong with you bill, possible misread on the meter, something like that.

Edit: forgot about delivery charges.