Modern gas fireplace inserts come in two distinct forms, vented and vent-free. Most inserts are vented models, which simply means they require access to outside air via a chimney, flue, or dedicated vent in order to carry away the vapors produced in combustion. When looking for a vented gas fireplace insert, be sure to note how much ventilation the insert will require, and match that to your existing chimney or flue. Otherwise, you'll need to arrange for a dedicated vent to be built for your new gas fireplace.
Vent-Free fireplace gas inserts, as indicated by their names, do not require any form of outside air access. These units operate at an efficiency level that allows them to run without a dedicated vent system, but they do require some considerations to be taken. These considerations include the fact that a vent free unit can bother individuals with lung issues, breathing problems, or allergies, and some may be sensitive to the faint smell.