In some jurisdictions, it is a building code requirement that dampers be permanently open for gas fireplace logs.
The reason for this is that using the gas logs with the damper closed would put deadly carbon monoxide in the house. With old-style wood fires the smoke will tell you the damper is closed. Not so with gas logs, where the burn is smokeless, but the invisible CO gas is still being produced. So, even if code doesn't apply -- safety just might.
This deficiency is usually noted by the Gas Company when re-lighting your pilots after a gas shutoff and restoration. But often, some home inspectors may note this missing device as an exception when trying to sell or refinance your home. For less than ten bucks and five minutes, you can increase your family's margin of safety when employing gas logs in your fireplace.
Installation is straightforward. clamps down on the damper blade. If the blade has a bend that precludes this, you can mount the stop as an interference jam - on the damper handle - so that the damper never gets closed. Here is some instructions from an original installation guide for a gas log set:
See pictures below.
Always use safety googles/safety glasses when sticking your head in the chimney space.
Resist the temptation to mount the clamp here! A strong gust of wind blowing into the chimney, coupled with the weight of the clamp can blow out a pilot light (if used). This is annoying, but not dangerous. You should move the damper clamp close to the hinge/pivot on the damper blade.
Here the damper stop clamp is on the edge of the opening.
Here we couldn't mount the damper stop clamp to the damper blade (the blade had a bend along the lip which made it wider than the jaw of the clamp). So we mounted the clamp on the damper handle in such a way that the clamp interferes with the handle travelling full stroke. The result is that we have the desired air gap at when the handle is at rest (the orange arrows indicate the gap).