The reason that the damper is supposed to be propped open is for 2
reasons:
1. If there is a gas leak with no fire, the gas will be drawn up the
flue via the natural draft, rather than drift into living quarters
and
2. If the damper is left closed on a wood burning fireplace, the smoke
in the room will let you know. With the gas fireplace, the 'smoke' is
not sufficient to let you know that the damper is closed.