One of the things that happens in the Temple of Elemental Evil is that, in order to ramp up the difficulty as you go down levels, the monsters get nastier. Instead of orcs you fight gnolls, instead of gnolls you fight ogres, and so on. The number of opponents stays the same or thereabouts. I imagine this works fairly well in D&D, but in GURPS it's a disaster. One of the first things you learn/are told as a GURPS GM is that piles of hitpoints do not long encounters make. That ogre might be intimidating to a level 1 party, but to a bunch of 100-pt characters it's a pushover.
Instead of a hitpoint economy, GURPS has what I'll call a maneuver economy. (I use that term to distinguish it from the action economy of 3.x edition D&D). What I mean by this is that combat is won by the side most able to do effective things each turn. At the same time, the ability to do effective things is fairly well constricted (with the exception of certain advantages or spells like Great Haste), so for non-exotic opponents it translates pretty well into 'number of combatants on your team'. There are exceptions, of course, which is why I put the emphasis on the maneuvers rather than the allies.
This is the case because, generally speaking, GURPS characters can't 'soak' attacks as well as high-level or high-hit-dice D&D tokens. in D&D you can win a war of attrition by having more to start with. In GURPS, not only is it a lot harder to have more to start with, but your combat effectiveness is tied on a turn-by-turn and overall basis to your damage taken.
Okay, so I haven't said anything new. Anyone who has GMed or even just played GURPS for more than a month or two understands the above, at least on an intuitive level. So, how to translate the D&D difficulty scale over to GURPS, especially when dealing with 250 point characters?
There's the simple answer, which I might go with: hire more NPCs. Instead of having two orcs, have five. Later on, instead of having two gnolls, have eight. The problem with this approach is that combat difficulty in GURPS is very difficult to judge, and it's a very fine line between outcomes, at least when judged by the numbers. (A smart party whose incentives are divorced from combat may very well find a way to avoid those eight hyena-men.)
The more complicated answer is to make your combat NPCs exotic; that is, give them traits that allow them to even the action economy out against multiple foes. (I mean traits like Extra Attack and Altered Time Rate, or even Compartmentalized Mind). The problem with this approach is that the opponents in the Temple are known (to me), and one of my goals is to hew fairly closely to the original, while still making it interesting to play.
I think what I'm going to do is, first, in the lower levels of the temple especially, pad out the numbers with less-nasty monsters. This works fairly well inside the world, too; it's quite believable that a small troop of bugbears has a posse of goblins at its disposal. (Plus, doing this might allow me to sneak in some more interesting fodder-type monsters, like gibberlings and xvarts (not to mention norkers). I don't have a feel for exactly how to do this yet, and I imagine each encounter will involve decision about how much to pad (and how to change room descriptions to account for the extras).
In addition, I like the idea that our well-known humanoids should still have some surprises in store (much like Peter D's hobgoblins, for which I sadly cannot find a more specific link). Therefore, I might decide all my githyanki are mana-dependent, or that kobolds are, due to their strange physiology (being the degenerate spawn of Elder Things) resistant to cutting damage, for two fictional examples.
Hopefully soon I can share more specifics. We game again tomorrow night, where we will hopefully finish up the currently running campaign, leaving the next-but-one Tuesday for the players to explore my version of Hommlet.