If it is a terra cotta clay flue then all that is required is an inspection to make sure that it is clear of debris. Burning gas produces carbon monoxide and water. Water vapor causes masonry, especially terra cotta to flake causing pieces to fall into the flue, adding debris in the flue and compromising the chimney structure. Gas appliances should always be vented using class b venting or other applicable gas vents suitable for your appliance. Also your chimney may contain squirrel nests, bird nests etc. all chimneys should have suitable caps. If your masonry chimney is free of debris do not have it cleaned as this will only cause more chipping of the weakened flue tiles.