A zero clearance (ZC) fireplace is a fireplace that has been designed to be built into a combustible frame wall. There are small “stand-offs” on the exterior of a ZC fireplace that that can be in direct contact with 2x4 or 2x6 framing. This makes for versatile and rather quick installations.
Do not get this term confused with the finishing requirements around a fireplace. There are definite clearance requirements, specified by the manufacturer, for every ZC gas fireplace. That’s why you will see brick, tile or stone around fireplaces. Maintaining minimum clearances to mantles and for the hearth is also critical. See your owner’s manual for specific details on your model.