Gandalf looked like a really loving cat. Russian Blues have a special place in my heart, as they are always so sweet. I know it hurts because it shouldn't have happened, and he should have had many more years of love with you. There is nothing you could have done, and you gave him everything he could have dreamed of. He was probably the happiest he's ever been in his life, when it got cut short. Thank you for giving that feeling of joy to him.
I know it's hard right now; it will just take time to heal from this. If you have more photos and you think it would help you heal, please post them. He looked like a super cuddly cute cat.