Yes, there definitely is a difference! These are NOT synonymous terms! A gas fireplace is designed to be built into a framed wall. This is done during construction or a remodeling project. There is an insulated shell around the firebox that provides protection against overheating any wooden framing members. Gas fireplaces can be vented horizontally through a sidewall or vertically through the roof by means of an aluminum, co-axial vent pipe.
A gas fireplace insert does not have this insulated cabinet or shell around the firebox. An insert is used to update an existing fireplace. An insert is slid-in (or “inserted”, thus the name) to an existing wood woodburning fireplace. Gas inserts use a co-linear venting system and must be vented vertically. Two, 4” flexible aluminum vent pipes are installed in the chimney. One of these is for the exhaust gases to leave the unit, the other for combustion air to flow into the unit.