[Varyon]

At least in 3e, part of the text explaining HP in DnD (I forget if it was in the PHB or DMG) had a phrase along the lines of "A dagger to the face is still a dagger to the face!" This implied that the GM was meant to ignore HP in cases where a character would "obviously" die, and just say "He's dead, Jim." Failing that, there was also the Coup de Grace option against helpless targets (which would apply to a bound hostage or suicide attempt), which meant an automatic critical hit as well as the target having to pass a Fortitude save (DC equal to 10+damage) or die outright.

In GURPS, unless using rules or Advantages that make characters particularly hardy (in which case the character not dying outright is Working As Intended*), the situations you describe are indeed quite likely to result in the character's death. There is a chance of survival, of course, but that's not terribly unrealistic. The girl with the slit throat is unlikely to die instantly (even with a high damage roll, 6 cut to the Neck Veins/Arteries will deal 15 HP of Injury, which can't instantly kill anyone with HP over 7), but is going to be bleeding rapidly, and will require Surgery or magic to save. If the noble with a crossbow to his head rolls low damage, that may mean the bolt went through a non-critical portion of his brain (normal people have survived worse). GURPS may overstate the probability of survival, due to it tending a bit toward cinematic "heroic realism," but you'll rarely get impossible results, unless in a setting where the impossible is possible... which again would be things Working As Intended.


*I was recently rereading some of The Zombie Knight Saga, and there's a somewhat humorous scene there where an enemy Servant commits suicide by repeatedly shooting himself in the head with his sidearm, emptying the magazine before finally dying. For context, Servants are basically undead that are bound to an incorporeal Reaper, and that become more resilient as they age (in an earlier scene a fairly old servant gets shot in the temple with a high-powered rifle; the bullet smashes itself against his skull, leaving only a small bruise). They can only be "killed" by destroying their brains, but doing so allows their Reaper to remake them from nothing. A common strategy in warfare is to capture Servants by decapitation and freezing and/or extracting the brain (the Reaper can't remake the Servant if the brain is still intact), so Servants in a losing battle will often destroy their own brain to escape (provided their Reaper is elsewhere).