I'm so sorry.
I had never lived on a farm or had chickens or anything and decided one day I wanted them so I ordered 5 day old chicks in November. Got them a coop and all outside, but turned our downstairs bathroom into a brooder because it was pretty cold and they stayed inside for probably 6 weeks. I got super attached to them, they were like family members.
My husband really wanted a RIR, so when ours were about 7 months old I saw a couple at a flock swap that looked healthy and we brought it home. I didn't know anything about quarantine or anything like that. Just introduced them to my girls, they got along great and that was that. My husband deployed to Afghanistan a few days later and about a week after that I noticed some sneezing. Eventually there was runny noses, some bubbly eyes, and stinky face to which I deduced from here the new bird brought in infectious coryza. I tried so hard to get them better to the point of giving them Tylan injections in their breast. I got a lot of pressure on here that not culling them would be irresponsible. I could get the disease on my shoes and track it to other people. All kinds of stuff. So without my husband I had to suck it up and cull them all myself. I did a lot of research on it, and it seemed the best way was to slit their throat because they would bleed out really quickly. I was really scared and apparently didn't cut deep enough on the first, it took her about 5 minutes to die and all I could do was hold her and sing to her. The rest I did properly and it went a lot quicker. But it was very traumatic and even now sitting here thinking back on it I am almost in tears.
So it's never easy, and I am very sorry you have to do this. I've never heard of gassing, that's a new one for me. I definitely wouldn't freeze or wring their neck. Though with it being that small you may be able to snap it's neck and have it die instantly.
I hope it goes smoothly for you!
*hugs*
