There are two good answers to consider.
Simply put, the best time to buy a gas furnace is before you need one. We’re fans of pre-emptive furnace replacement. First, you won’t get stuck without heat in extreme winter weather for a few days or longer.
Secondly, you can take the time to decide what you want in terms of furnace efficiency, performance and cost. A thought-through, informed decision is probably one you’ll be happy with down the road.