Hahahahaaa okay so Gansey is afraid of a lot of things. The three biggest ones are hornets/wasps/bees (which will kill him), his people dying, and his friends hating him for dying. So there will be lots of hornets in his hallucinations, and he will see lots of dead people, mostly the other Ravens, but also possibly Blue, Malory, his sister, and some folks from Box.

All of those threads about other people dying will involve some pretty gristly things, including a suicide attempt, physical abuse, murder, and some other terrible things. Also a lot of spoilers for his series. These things aren't occasional fears, either, they're things he dwells on, so the likelihood of Gansey being almost constantly stalked by some fear or another is pretty high.