I'm so sorry to hear about your dear little Gus. It sounds like you did absolutely everything for him - good vet care, love and support. Sometime they just don't make it.
I just lost my youngest chin Moki who was 9.5 on Monday very suddenly a month after stitches in her arm had healed. The hole in my heart is so real and so gaping, I didn't think I could go for a few hours without breaking down in tears. I have 2 wonderful happy chins who are 11 that are helping me grieve and try to move forward.
You loved your Gus and he knew that. Its so hard to let go of the memory of these painful deaths, but try to think about the wonderful, amazing life that Gus had with you up until that point.
Huge hugs to you and raisins and dust baths to your little Neo.