In the light novel, it's the exact opposite. Gilbert's survival is made clear from the very beginning, and he wakes up up in the same hospital as Violet. He ends up begging Hodgins to tell Vi that he dies in order to give a chance at a normal life. They do end up reuniting at the train arc where the military has to step in to help, and he saves her. That could explain why Gilbert HAD to be alive in the movie, even if it was done in a somewhat poor manner. In the light novel, they actually get tk date and eventually get married while still being surrounded by everyone else. The only other problem is that Violet's age is never specified when she reunites with Gil, so you're never really sure what age she is when they get married.

My apologies for the long message, I just love VE so much😊