I don’t hate the idea of Belle growing old and dying as long as it’s part of her happily ever after with Rumple. It’s very romantic in a way like the scene in Highlander with Heather dying in Conner’s arms. But hopefully with a better make-up job or preferably an older actress cast to play elderly Belle.
As for Gideon’s age relating to all the other timeline issues with Henry and Lucy…at this point the timeline is so wibbly wobbly that it makes River Song’s timeline in Doctor Who look simple. I don’t know that there even is an explanation to be had. Gideon will be twentish years old because the actor they hired is in his twenties.