I'm so sorry for you and Gus. It is the most horrible diagnosis you can get. My Jack Russell Oreo passed away 3 years ago at the age of 14 from a combination of cancer and old age. She lived with it for 2 years after she was diagnosed, and it was hard to watch her decline, sleep all the time, loss of appetite, and not being able to go up and down stairs anymore.
Enjoy Gus, spend time with him, and take lots of pictures with him and your family as keepsakes and memories, and when the time is right , you will know when it is time to let him go.