Gas fireplace venting issue
Gas , oil, and wood need air( oxygen) to burn , then the flue gas can be vented. So , let in some air, A K A -open a window. When I lived in a colder climate , I always provided an air inlet , generally a duct pipe from outside to near the furnace, or fireplace in this case. Newer furnaces are generally required to have an ejector fan on the exhaust vent because newer houses are well sealed and it is difficult for a natural draft to work. The lower the outside temperature , the more the problem.