I'm so sorry for both you and Gizmo. I'm not going to reiterate what everyone else has said, it's all true, but I'd like to impart one piece of advice to you:
I've had dogs and cats over the years and every time I've brought them to the vet's office for their "last visit". In 2014, when my cat Atticus, who I loved more than anything, was losing his battle with cancer, I knew his time was short and he would have to make that "last visit" too. Now, my vet is/was great, and he was one of her favorite patients, but this time there was no way on his last day that I was going to take him out of the house on a bumpy car ride and have his last moments on a cold table under bright lights in a noisy office.
I got in touch with a company called "Pet Loss At Home" - the name pretty much says it all - and made an appointment to have a qualified representative come to my home for that last visit. My Atticus left me at last in a quiet room, on his comfy couch and blanket where he slept every night, with my arms around him. It was the best choice in a bad situation, and I'll do it again with all my pets in the future.
It's going to hurt for a long while and you'll always miss him, but it gets better with time. Since we're all sharing photos, here's my boy.
