One of the things I wanted to emulate in my Felltower/Cold Fens campaign was the old "need silver or magic to hit" rule from D&D. Not for any reason except it added some challenge and otherwise helped match the feel of the games I was pulling concepts and ideas from.
Christopher Rice wrote great post on this subject the other day. Which prompted me to write down how I do this myself, mechanically, in my own DF game.
So, how to do this?
Since GURPS doesn't tie "to hit" with damage directly, you don't want to directly port the concept of "silver or magic to hit." Hit all you want, it might even do Knockback, but it won't cause any injury. Plus, in my games, it's Puissance that counts.
I went with Damage Reduction (GURPS Powers, but especially GURPS Supers, p. 146). Also, I make use of the suggestion for Cosmic from GURPS Powers p. 119 for allowing Cosmic, +50%, to drop minimum damage from 1 to 0. Finally, I couple this with an Accessibility limitation that restricts its effects versus certain attacks.
For example, one of the critters in my DF game has the following:
Damage Reduction/10 (Not vs. Puissance +1 or better)
Hit that guy for 20 penetrating cutting damage and he takes 30/10 = 3. Not likely to go down from even hard blows, but eventually non-magical strikes can wear it down or something like Mungo the Giant Troll can pulverize it in a handful of hits.
Another is basically immune to non-magical damage, and thus has:
Damage Reduction/100 (Cosmic, minimum damage 0; not vs. Puissance +1 or better)
Hit that guy for 20 penetrating cutting damage and he takes 30/100 = 0.3, dropped to 0.
Still another can only be hurt by the greatest of weapons or by bare hands, because godlike invulnerability often forgets to cover all of its bases.
Damage Reduction/100 (Cosmic, minimum damage 0; not vs. Puissance +3 or unarmed attacks)
I did say "silver or magic." For those, just add "silver" to the exclusions. It's up to the GM if silver coating is enough; if not, perhaps it just cuts the reduction down a notch, so /10 goes to /5. Solid silver should always do the job.
What else? I find that DR only vs. non-magical attacks will work okay, too, but generally means you can still get splattered by a sufficiently hard non-magical blow, like from a dragon or giant. That's okay, too - high HD critters could do that in D&D, too, according to the 1st edition DMG, p. 75.
I also couple these with Injury Tolerance and a variety of Vulnerabilities and Weaknesses and things like Unkillable, Regeneration, and Supernatural Durability to get the "right" mix of toughness and survivability.
What about spells? Mostly I stick direct-damage spells under "magic." Dehydrate, Frostbite, Deathtouch - they all count, generally, as magical damage. Ones that inflict it indirectly via an effect or missile (Create Fire, say, or Lightning) are mundane damage. However, many such critters have other vulnerabilities, so you might have a creature basically immune to non-magical weaponry and many spells, but which is vulnerable to fire. For them I'll expand the accessibility limitation on their Damage Reduction.
Just harder to kill? You can also make creatures just less likely to die from such weapons. Supernatural Durability (Achilles Heel: Silver or Puissance +1 or better weapons) is one way to go, as is Unkillable with the same kind of limitation.
And that's pretty much how "silver or magic to hit" works in my game.