It depends on what kind of circuit you're switching. Most gas fireplaces are a low voltage circuit that needs to be closed to turn the fire on, or open to turn it off. If this is the case, you will need to get an isolated contact closure like a zrw113, but you will also need to get 120v into the same box to power the switch. Beware that code requires that all the wiring in the box be rated for the highest voltage in the box - typically, we use 600V rated wire for this, but you can get by with anything rated for 250V.
If your switch is actually closing a 120V circuit to the fireplace, any zwave switch should be fine, but make sure you have a neutral available at the switch. Gas fireplaces are easy to work since you usually have voids in the wall around the fireplace to pull wiring, and you have 120v available under the fireplace if you need to extend it to the switch box b
Vera1 (1.1142), Vera2 (1.1182), VRI06 (12), VRS15 (3), VRS05 (2), VRF01 (2), VRCS4 (2), ZRW113, ZRF113 (2), 45602, 45603, TZMT400 (2), FE599 (2), 99100, Thinkstick, Harmony 890Pro (2), Harmony RF Extender, Nevo S70, Nevo NC-50, Minimote, SQ Remote, SQ Blaster, EtherRain-8, Cliste ActiveRFID, TED5002