What's this black handle on my fireplace? It moves up and goes left
I found this 5 year old unanswered question because I'm in search of a similar question for my own fireplace insert except that I'm looking for a rod.  I bet this is the damper control lever (yes, not the same damper as the one at the top center of the stove.  This damper should be in the in the flue- and I bet this little rod both moves and pulls out (pulls out to close the damper and moves to hold it closed).  I should add that the damper itself (the disk) will never completely close, it's missing approx 1/4 top of the circle where the joint is, probably for safety purposes so perhaps it is for gas as well.

These inserts are so old, mine is a 1970 Fisher Grandmother stove and boy is this a wild goose chase!