The heat from a gas fireplace is normally limited to one room, while central heat covers the whole house. A gas fireplace with blowers that force the heated air into a room is more efficient than one without, but the heated air takes time to move to other areas, and may never reach another floor of the home.
Gas fireplaces, at their best, are still an inefficient way to heat a home. Much of the heat escapes up the chimney and is wasted. A central heating system conserves the heat by forcing it through duct work to ensure that it arrives where it is needed.
Central heat is almost always found in a home in moderate and colder climates unless it is an much older dwelling. While a gas fireplace is easier to build into a new home, it can also be retrofitted into an existing home.
A newer central air system may slightly add resale value but in most areas of the country it is expected that a home will come with one. A gas fireplace adds beauty and heat to a home and adds to the resale value as an optional feature.