The phrase "gut punch" means something that emotionally or physically hits you very hard or catches you off guard, leaving you feeling shocked, devastated, or deeply hurt. Full definition
The quiet that settles over the group's strictly controlled daily routine is short lived, leading to a real gut punch of an ending. (film.avclub.com)
If you can connect with the hopeful girls, perhaps the film will have the intended emotional gut punch for which it strives. (moviereviewsfromthedark.com)
Nothing would prepare me for the emotional gut punch in its second episode. (darkstation.com)