Fuel is fuel, the more you burn the more money you will be throwing out the window(s). I would strongly recommend switching out the windows. You do not have to do them all today if you can't afford it, one by one as you can (Though that is more expensive in the long run).
A few temporary fixes to the heating issue:
1. Big house with two people in it, then select those rooms which are used less to shut off from the whole house heating. Such as closing the vent(s) to those rooms. Warm air will still get in there, but not as much thus the room will not freeze in the winter, but it will not be comfortable either. older houses had doors to all rooms - not just for privacy, but for the functionality of closing off parts of the house and concentrating the heating in a few rooms.
2. Insulate the windows: We have those louvered windows, you know the type narrow panes of glass that seal up as about as well as a sieve. In the winter I go through the house and on most of them I put up the same sort of vinyl plastic that is used in clear shower curtains. It is carried at fabric stores, comes in several thicknesses. It isn't too difficult to make a frame of 1/2 or 2/2 that will fit snuggly in the window sill. It will act like another pane of glass, giving a "dead air" space to help insulate from the extremes in temperature. It is about as effective as a "storm" window which usually does the same function. Not as efficient as double paned windows. That is my first line of protection, my next is....
3. The Victorians had heavy thick curtains made out of velvet and other wonderful plush fabrics. It wasn't just a style they were very functional as well. Victorian window dressing was to provide another layer of insulation over the most inefficient insulator - glass windows. If you look at pictures of old Victorian Homes you find that the drapes went from floor to ceiling and went all the way against the wall on either side. Most of the big bay windows and other big windows where built in a manner which allowed one to curtain off the whole area "sealing" the small room from the rest of the house. Try hanging heavier curtains, or in some rooms cover the existing curtains/drapes with a comforter or blanket. Works best at night when you are not using the daylight.
Gas fireplaces - ALL fireplaces - are not designed well for heating. Most of the heat goes up the flue/chimney (even with those inserts), most often the fire place has to run for a fairly long time to heat up the surrounding brick. Like for days at a time, then the heat from the brick radiates into the house.
Ben Franklin was a pretty smart fellow, he figured out a better way at using the heat of a fire more efficiently, he invented the "Franklin" wood burning stove which was a "fireplace" cast in iron but sat inside the room - you could watch the fire by opening up the front iron shutters, you could also cook on them, heat water, etc. The flue (a tube of metal) would stay inside the house radiating more heat on the way to vent the smoke.
Types of Fireplaces means they do different things:
A decorative fireplace usually has a wall facing outside. A fireplace for heating would be in the center of the house where the brick and masonry are radiating the heat into the house. Kitchen Fireplaces (when they still cooked with real fire) would be on an outside wall to radiate the cook fire heat in summer outside - Same principle for the brick insert for a wood burning cook stove, to move the heat from the kitchen to the exterior. Imagine the heat of the kitchen in August if we still cooked and heated water with a wood or coal fire.
In truth a gas fire place will not be used much. Think you are having a heart attack over efficient modern heating, wait until you get hit with a natural gas or propane bill when you use a gas fireplace for a month. I am willing to bet that you will use it for one month and figure that it was really a bad idea after all. That is why they went out of favor and wood came back the price of fuel wood is far cheaper than natural gas AND propane.
To blame the huge old windows may be erroneous, that is the obvious - the not so obvious is that older houses do NOT have wall Insulation if there is insulation the attic or crawl space above, it is most likely thin or if it is blown insulation has compressed over time.
Heat leaves the house in several ways:
1. Glass panes (number one on older houses).
2. Poor roof insulation (right up there near the panes of glass, in many older houses above windows).
3. Poor air seals: Around windows, doors, minor cracks and crevices, up through the flue/chimney, out vents,around outlets on exterior walls.
4. Through uninsulated walls (most common in older homes)
5. Through the floor. Being and older house I am willing to bet it is not on a slab (which are wicked horrors when it comes to heating and cooling and are not used in the north eastern states much) Most likely a nice crawl space or if you are really lucky a basement/cellar. Floor insulation will increase the over al efficiency of a house greatly.
This Article goes into more depth on this subject.
And This Site offers more of the same.
Personally I would assess my living situation. If you own the home and can't part with it (some people do believe in setting down roots and will plan on spending the rest of their life there) then you should start looking at a long term solution, which means getting in touch with energy efficient consulting professionals, start with your local power company/gas company. Have them come out assess the full situation and then work on a step by step methods of improving the energy consumption of the house. Many energy companies (electric, gas, etc) have incredible home owner rebates and incentive programs which will reduce the cost of improving your home.
A gas fireplace is not the best first step at a solution, if anything it will be a waste of time and money and will add to the fuel bill problems.
David