A fireplace is heavily a décor element, and thus intended to look pretty, even at the expense of efficiency. Some gas and electric ones will even have speakers built-in to provide the sound of burning wood. Typically, you can adjust the mixture in a gas fireplace to be more efficient, though doing so will sacrifice much of the décor element.

You can also get gas space heaters, which are intended to produce heat without looking pretty, and will generally be adjusted for a high efficiency burn.