Not gruesome as such but the worst death is definitely starvation! Like imagine being trapped in a cave somewhere with condensation on the walls but nothing to eat. And u slowly just waste away over the course of a few weeks. And u start going more and more insane as ur brain breaks down bit by bit.
I read a book not too long ago called The Bunker Diary where the character (16ish?) and a little girl of about 9 die like that.