Of course, Canelo Alvarez can still fight even if he losses to GGG. Losing to GGG doesn't mean it's now the end of his boxing career. I doubt Dmitry Bivol's camp will still be interested in taking the challenge from Canelo. There's no Title at risk and Bivol wants to add some title to his career. Canelo should go back to his comfort weight class and fight those top on the ranking there.
Until Canelo doesn't have any title at stake, Bivol will just ignore him for a rematch. But first, he needs to win against GGG. He won't stand a chance at Bivol again if he won't be able to beat the old and veteran GGG.
And about Bivol, even if Bivol right now seems to be interested in fighting for yet another belt we must not forget professional boxers also fight for money, and I have little doubts his fight against Canelo was the fight which made him the most money, so I do not think he will be against a rematch if it is ever proposed to him.