Fogmire is a very linear and somewhat random dungeon that nominally acts as the conclusion of Here There Be Monsters (HTBM) in STAP. For my purposes, it has advantages in that its short, straightforward, and filled with interesting traps. It has the disadvantages of being so linear that adding any branching or loops to it would require completely rewriting it, and the one monster to a room problem that is so common with D&D3e adventures.
I can deal with the linear nature of the dungeon by pretty much ignoring it and looking forward to the adventure, Tides of Dread (ToD). ToD is a mini-sandbox, exploring the Isle of Dread while preparing for a pirate invasion. So this silly linear dungeon is fine; the PCs are going to be under a lot of time pressure (their allies have been captured by teleporting demons and are being tortured). A straight dungeon is simple for them, and instead of worrying about which way to go, they can focus on resource management (prayers, potions, HP, time) and not dying in the traps.
The one monster to a room is more of a concern. 5 PCs, plus 1-2 allies, versus a single monster is going to go poorly for the monster in GURPS. Okay, it goes badly in D&D, too, but it's really obvious in GURPS. The choice of monster acerbates the problem. A lone naga wizard is just going to get shredded, and disguising itself as a zombie "to make it look less threatening" isn't a plan, it's insanity. The mob template of gorillas is just a boring melee monster.
My simple solution to these problems is to combine two of the rooms, change the mob template into a bunch of gladiator apes, and move the naga wizard down the hall to the Shrine. A pair of teleporting demons, grabbing people and pulling them onto the upper level of a room with two levels, should distract the PCs plenty while a few waves of a dozen or more gladiator apes come charging in. The naga wizard works better at the end of a long hall filled with traps than at the front of it; the fact that she has another teleporting demon martial artist to play bodyguard should also drastically increase her lifespan. The fact that the PC's allies are slowly dropping toward a pit of fire should also nicely divide their attention, though honestly only for a round or two.
Notes on the conversion to GURPS are here.