Really sorry for ur loss..I know damage is unbearable but your dog lived for good 91 human years and in later stages of life any of the major organs function gets compromised due which signs are reflected and in this case it was not simple gastroenteritis but nervous signs were also accompanied like banging head and getting scared and it's not the same story for every dog. Bt it's rightly said in English "dogs have a difficult death". May he rest in peace.