Mike, I seriously doubt that you'll need 20 amps to run a forced-air gas furnace. Mine is rated at 9 amps, and heats an old 1200 square foot farmhouse with capacity to spare.

9A x 120VAC = 1080 watts. In theory, a 1200 watt generator should be able to handle that (but nothing else). In practice, though, there's a big momentary current draw when the fan motor starts. If you read the fine print, you'll find a lot of generators rated at 1500 watts are in fact 1500 watt peak, and 1200 watt continuous. And 1200 watt generators are 1000 watt continuous, etc. Buyer beware.

BTW: a couple of gas fireplaces plus an efficient wood stove saved the day after my parents had a serious garage/house fire in -35C weather. All power was knocked out for two days, but with a bit of antifreeze in the traps and these heaters, we kept things from freezing up and causing more damage. The heaters were natural gas, and were the type that had didn't require electricity to run.