@the Tarrasque and creatures like it
I actually find that, if you want to call the Tarrasque (and creatures like it) a puzzle encounter, it's at least of the right sort. It tends to reward creativity more than system mastery/memorizing the monster manuals. The Tarrasque is intended to be a melee monster of epic proportions -- it's really solid in the deal and take damage department. But, it's a big dumb brute without any exotic abilities other than being immune to a bunch of stuff. Hence, it's well-known vulnerability to stealth, flight, etc. Really, to anything that doesn't challenge it in its bailiwick.
That's a damn sight different than the Troll's sort of obscure -- to anyone unfamiliar with D&D -- regenerative properties and the like. The ways to respond to the Tarrasque are intuitive, i.e., they benefit relatively little from system mastery. Contrast that with Shivering Touch + Dragon's or other low Dex monsters.
I happen to like well-informed players, maybe b/c I am one, and generally trust players to know their mechanics. But, that's in full light of gentleman's agreements, etc. I happen to like GURPS, too, or I did 100 years ago when I played it. I wouldn't say that it's not a math game or an exploitable one, though, just FYI.
Also, GURPS, like nearly every other game on the market (anyone let me know if you have any counterexamples) takes a lot more labor to run than D&D. The existence of the Monster Manuals and even some vague, crappy guidance as to what to use is a tremendous boon. I was sort of tempted, largely by nostalgia, to finish up my conversion of Rifts, or something Rifts-inspired (giant robots + magic + whatever + action!), to M&M and it stalled b/c I realized that in order to run the game you would have to create every encounter from scratch, which can get really tedious.