The lethality of GURPS can be a positive and a negative. As a positive, it makes for tense combats, adds real risk to decisions, and gives a lot of believability to injury. As a negative, it totally sucks to have your character, who you worked hard on to make, just roll badly and die. That tension - characters take effort, but can die easily - makes for the feel of the game. Still, no one likes to die, so let's look at how easy or hard it really is, and ways to avoid it at the last moment - the roll not to die.
This is at least partly inspired by the death toll in my fantasy game, and by Douglas Cole's excellent look at skill levels in GURPS. Also check out Doug's look at the pricing of HT in GURPS.
In GURPS, getting grievously wounded is pretty easy. Most fights in GURPS, in my experience, turn suddenly when a telling blow gets through and changes one combatant from "just fine" to "potential death spiral" in a single moment. Miss a defense roll, watch the other guy roll a critical hit (which gives you no chance to defend), or get hit by surprise or an area attack and you could go right down.
Looking only at GURPS Basic Set. Shock (p. B419), Major Wounds (p. B420), Crippling injuries (which are also Major Wounds) (p. 420-422), Stunning (p. 420), Bleeding (box p. B420), Hit Location effects (p. B398-400), Consciousness rolls (p. B419) - all of these things make it hard to stay above -1 x HP (or below) and not have to roll to see if you don't die.
Not only that, but you don't get a lot of HP. Everyone starts with 10 HP, but getting even twice that is somewhat uncommon except in high-powered play (where it's very common), and you don't get more unless you keep buying more.
But, especially in 4th edition, it isn't easy to actually die.
First, you don't automatically die until you've gone down to -5xHP . . . meaning you can take as much as 1 point shy of 6 x HP and still live.
Second, there are way to deal with even missing a HT check against death. In 3e, if you missed a Death Check (a common but not-official term of art for the HT roll to see if you die or not), you died. Have a 15 HT and roll a 16? Oooh, time for Resurrection or superscientific revivification or a new PC. But 4e has Mortal Wounds, which means you can miss the roll and still be alive enough to get better, given enough medical or magical support.
This significantly changes the odds of death.
Let's look at death, the odds of death, and how much HT you need to reasonably avoid it.
The first thing you want to do is, avoid getting to -1 x HP or below. But once you are there, you need to start rolling to avoid dying. GURPS Basic Set p. 419 makes most of these pretty plain, and I won't re-hash those rules here. They are also on p. 29 of GURPS Lite, which is FREE, so go get it!
How many times do I roll?
One thing that GURPS Basic Set p. 419 doesn't make crystal clear, IMO, is how often you roll against HT to not die. For example, if you have HP 10 and take 30 damage, you are at -20 HP. Do you roll once for -10 HP and another time for -20 HP?
Yes, you do.
What makes it clear is the example of Bruno under Hard to Kill (p. B58), who takes 45 damage and passes two death check thresholds, and must roll twice.
How do I recover from a Mortal Wound?
Surgery skill + time, or the Stop Bleeding spell (no, seriously, I didn't know that second bit until recently either.)*
How much HT do you need?
Read "HT" as "HT including any bonuses." Advantages like Fit (+1 to HT rolls) or Very Fit (+2 to HT rolls) and Hard to Kill (+1 per level to HT rolls against death, a miss has some special effects) make it harder to die. Just add your HT plus your bonuses when you look below. Ex: A HT 12 guy with Very Fit and Hard to Kill 1 rolls against a 15 not to die.
Death checks are very rarely penalized; while unconsciousness checks get cumulative penalties for injury Death checks don't. So to not die you really don't need a huge amount of HT, or worry about counteracting penalties.
Below is the HT, the chance to live (without problems), chance to live or be mortally wounded (with potential issue during or post recovery, per B.423), and chance to die outright.
Percentages of success are taken from p. B171 for consistency.
Remember that in GURPS, a 17 is always a failure and an 18 is always a critical failure (not a big deal on a death check) (per p. B343 / GURPS Lite p. 2) However, it could make a good ruling that if you have a total HT of 16+, a roll of a 17 is a Mortal Wound and a 18 is death. You could further rule that if you also have Hard to Kill you appear dead (although you're still Mortally Wounded). However, an 18 would still failure regardless of your HT, HT bonuses, advantages, and/or Mortal Wounds.**
HT 9 or less: No one takes these; suffice it to say it's easy to die and you don't want HT 9 or less.
HT 10: Roll against 10 to live; a death check at 11 or 12 means you are mortally wounded. 50% chance of being okay, 74.1% chance of being either okay or mortally wounded. 25.9% chance of death.
HT 11: Roll against 11 to live; a death check at 12 or 13 means you are mortally wounded. 62.5% chance of being okay, 83.8% chance of being either okay or mortally wounded. 16.2% chance of death.
HT 12: Roll against 12 to live; a death check at 13 or 14 means you are mortally wounded. 74.1% chance of being okay, 90.7% chance of being either okay or mortally wounded. 9.3% chance of death.
HT 13: Roll against 13 to live; a death check at 14 or 15 means you are mortally wounded. 83.8% chance of being okay, 95.4% chance of being either okay or mortally wounded. 4.6% chance of death.
HT 14: Roll against 14 to live; a death check at 15 or 16 means you are mortally wounded. 90.7% chance of being okay, 98.1% chance of being either okay or mortally wounded. 1.9% chance of death. Note: Maximum HT needed to live on a 16 or less including Mortally Wounded.
HT 15: Roll against 15 to live; a death check at 16 means you are mortally wounded. 95.4% chance of being okay, 98.1% chance of being either okay or mortally wounded. 1.9% chance of death.
HT 16: Roll against 16 to live; technically you can't be mortally wounded anymore because a 17 or 18 still fails. 98.1% chance of being okay, 1.9% chance of death. Maximum HT needed to live on a 16 or less without needing to resort to Mortally Wounded.
However, Sean Punch appears to state that on a net HT 16+, you should treat 17 as a Mortal Wound, 18 as death. So in this case it would be a 98.1% chance of being fine, 99.5% of being fine or mortally wounded, and 0.5% chance of death.
HT 17+: Doesn't matter, see HT 16.
So what does this mean for my character?
Basically, the most HT + bonuses you need to avoid death is 16. The most you reasonably need, assuming you're willing to suffer Mortally Wounded as a condition, is HT 14. Total. That gives you the maximum chance of not dying while still giving you the least cost to do so.
How you get there depends on your other goals for your character. Hard to Kill at 2/level is the cheapest way to avoid death, but that's all it does. Fit and Very Fit are good if you want a bonus to the roll and want to recover lost FP faster (or lose them more slowly, too). HT is best overall because it gives you the roll, plus marginally improved Speed and more FP, too, but it also costs the most (10/level).
But if you ever wondered why 4e monsters have HT 11-13, and most templates for really hardy types rarely exceed HT 13, it's because that really does give you an excellent chance to avoid death.
What if that's not enough for me? 1.9% Sounds high . . .
Get enough HT + Fit/Very Fit + Hard to Kill to not die, buy Luck, and save one use for a re-roll. Or buy Luck with a limitation that restricts it only to HT checks not to die. And try to avoid getting hit in the first place.
Are these odds realistic?
Wrong question, since GURPS put a slightly heroic blush onto PCs. It makes them believably fragile but also throws the odds of dying a bit in favor of the character. It's a game, after all, and matching easy death with detailed character generation is a mix that makes death especially harsh and not-fun.
Doesn't this mean GURPS characters are too sturdy?
No, not really. It's depressingly easy to go from HP to -5 x HP and just die. It's depressingly easy to bleed out out or be too injured to escape. You can drown or suffocate or suffer a heart attack from a truly horrid supernatural attack or fright. And, per p. B423, you can just die automatically from a slit throat, etc. if you're helpless and time is taken. Even HT 16+ and Hard to Kill won't help you there.
Good luck not dying!
* As a house rule, I also allow the Healing advantage to do this.
** I like this and I'd run it that way.