Sometimes with a really deep gut hooked fish, you'll injure the stomach of the fish during the fight, so much so that it's lethal, because you're essentially winding him in by his innards. In those cases, even after hook removal, the fish won't survive. He's done before he even gets in the boat.
But my point is - always remove the hook via the method above. At least then you've done your best to increase his chance of survival. But even then, you'll still lose a few gut-hooked fish.