I emailed Andair asking for size and material specs for the O-rings for the GAS375 Gascolator. Jenny at Andair replied: "The ORO375's are made from Viton 75, the dimensions are 39mm diameter x 2mm thick".
That is what I ended up with, but I don't like the fit. I think a 2.5mm X ? (36mm) may fit better. I just haven't gotten around to trying. The grove is 2.5mm X 39mm. I believe the ring needs to be smaller than 39mm to avoid my original complaint.
I have updated my original post so that those searching the forums will not have to read the entire post chain. I have found that a 2.5mm X 36mm is a perfect fit. It fills the grove, fits tight enough that it does not get caught when installing the bowl, and provides a firm seal - unlike the Andair o-ring, which slips in so easily that I wonder if it is even touching.