De-gassing tool for bottles...

Couldn’t you just screw it on the gun part way until air starts coming out, stop there and let it all run out? That’s what I did, a week ago. 

You will also need a 3/4-2” spanner wrench. I just bought a good (I think) one, (not here yet). When you use the spanner wrench, MAKE SURE you stay on th metal band, NOT the carbon fiber or you will break the neck of the bottle.
 
Human makes one if they are still selling it and I say that because it will push the valve assembly apart inside the bottle valve necessitating the need to remove and repair.

However it works fine for degassing just don't use to refill.

Bangel and Kirkwood make one the works for refilling and I assume degassing, hope I got their name right, they are a UK company.

Also I believe you can just slowly unscrew your bottle till it starts bleeding and leave it at that point and let it bleed down.
 
Couldn’t you just screw it on the gun part way until air starts coming out, stop there and let it all run out? That’s what I did, a week ago. 

You will also need a 3/4-2” spanner wrench. I just bought a good (I think) one, (not here yet). When you use the spanner wrench, MAKE SURE you stay on th metal band, NOT the carbon fiber or you will break the neck of the bottle.

Yeah you can do this OP.