Scotty, what you did was a grace for Gus. Doing the right thing for someone else isn't always easy, nor is it always something that will make you feel good. Your loss will heal in time. Just know that we understand your grief, we have felt the same, we know the pain, and we give you our sympathies for your loss.
It is a bond, an un conditional imperative that we form with our pets. And in the end it is honored by both the unconditional love we have fo each other. You have upheld your part by giving him a good death with no pain, he left before the quality of his life made living a burden. He left knowing you loved and were with him.