How can I close a gap between ceiling drywall and a furnace flue?

My gas furnace is located in my laundry/utility and is normal(~80%?) efficiency with a Type B Flue. It goes up through the drywall ceiling into my attic(1 story) and out the roof and I verified that there is no insulation in contact. There is a ~1" air gap in the drywall which is a draft leak and worse is pulling in a significant amount of combustion air from the attic.

I’d like to seal this gap and can’t quite make sense of the best way to do this. I understand that no contact with combustible material within 1" is code for type b vent but what is defined as combustible is not clear.

The closest thread I can find here is this one for a range vent How do I air-seal a range hood flute?

But I can’t easily slip on the collar so I was thinking I could cut a slit to bend it onto the pipe at the ceiling and then aluminum tape or firestop caulk the cut once installed.

Could I instead just caulk the gap with 3m fireblock which is noncombustible? Or is setting drywall compound considered noncombustible?