About 95% of my work these days is repairing gas fireplaces, and the remaining 5% gas furnaces.
I've found that while there are lots of people installing gas fireplaces, relatively few people are interested in repairing them. Often, the fireplace shops that offer service do so only on equipment they sell, leaving a lot of people out of luck.
So I've been working myself into specializing in gas fireplace repair over the past several years.
I used to work for a gas utility doing repairs on gas equipment, but I left them three years ago in favor of devoting my time to my own repair service. This has proven to be an excellent decision.
I'd be interested in whether other service techs are specializing in gas fireplace repairs, or doing such repairs occasionally among other work.