Musty Smell Coming From Gas Fireplace

Hi everyone, for the past couple of months I’ve been trying to figure out where a musty smell has been coming from in the living room. Smells like an old couch that may have had something spilled on it. Would come and go but we could never figure out what it was. Finally got determined to figure it out and found that it’s coming from the gas fire place opening. It’s not natural gas smell. Any thought on how to get rid of it or what to check for?