Gap between 6" dia wall thimble & 6" hvac pipe
B-vent is what should be used for exhausting gas appliances and most likely why you have a gap in a 6" escussion with a 6" pipe in it. Notice the ID and OD for B-vent. If its just an air handler for a bath fan use a flapper vent.



AMPG's PDF

Now you can fill the new 1/8~1/4" gap with an approved high temp caulk.