When I hear Gas Euthanasia I guess I'm thinking of something different.

When my Dexter was very ill and it was time for him to go, my vet gased him using Anesthesia. When Dexter calmed down and started slowing down, he then injected either into the abdomen or heart (I can't remember, I believe it was abdomen). This way, Dexter didn't "feel" the injection... he just felt himself drifting. Once he was injected, he did die almost instantly, but I was allowed to be back there with him through it all. I held him for 2 hours before he died, and 2 hours after. It was so sad to see him drift off, but yet so peaceful because I know he wasn't in any pain when he died.

I know this isn't Gas Euthanasia... obviously.

What method is this? There was gassing involved.