Look for a white paper wrapping on pipes or insulating cement on the furnace. If you see it, keep in mind that you are buying into a future asbestos abatement issue. I had a gravity furnace in our first house and had no problems. Basically there is no fan to malfunction, so as long as the pilot, thermocouple and gas conroller work, you have heat. These are not efficient combustors and waste about 40 to 60 percent of the potential combustion heat. The ducts are large to allow convective flow, and heat is often distributed to the upstairs by holes in the floor covered by grates; and room to room via transoms. There are more inovative replacement or supplemental heating systems available all the time. You may find they are both more efficient and less costly than conventional systems requiring new ducts.
When buying a house like this you need to understand that the furnace is only one part of the system. Chances are you have zero to minimal insulation. There will be major infiltration leaks at windows where counter-weights are in walls, and at doors that are poorly weatherstripped. Each improvement you can make will lower your operating costs and increase comfort. If you are going in with your eyes open to the necessary work and significant costs involved with modernization, go for it. Be sure to adjust your offer to give you economic leeway to address what needs to be done.