I was thinking back about some old posts over on No School Grognard, especially this one:
Castle of Horrors Session 7
It features a "mook rush" that doesn't have much of an effect.
That's been my general experience with mook rushes in dungeon fantasy style games: although the maneuver economy implies that three to one odds should be good for the mooks, the mechanics of actually getting all their units into place and attacking at the same time never quite work. Mooks in the front line get killed too fast, and their replacements end up with the unviable options of Move (and not attack) or Move & Attack (and miss) or use some kind of Attack maneuver with a Step of 2+, which exposes them to deadly counterattack on the next turn, thus repeating the cycle. All of which lets the PCs heroically stand against a tide of mooks pretty successfully, as long as the PCs stay together in a mostly anchored line so the mooks can't envelope them.
In a game like AD&D, if the mooks get in some lucky shots, they get in some lucky shots. Even "hits on a natural 20" means 1 in 20 mooks deal their damage to a PC. That adds up, since healing is a finite resource. PCs can be coaxed into using finite magical resources to destroy the mooks, too. In a tournament-style game, one-shot, or a megadungeon delve, rest simply isn't going to happen. Lost HP means expended healing spells, and that means a finite resource is consumed or lessened.
In GURPS, that's not really the case. It takes more like a 3 or 4 to get in a PC, and that can be mitigated with Bless if you allow it and Luck in all other cases. It takes a lot of lucky blows to hit a PC, then the strike needs to penetrate DR, and then and only then are resources consumed . . . perhaps. Most of the time, it just gives the PCs some FP loss, if that, and some EP usage they can recover easily. In a DF game, defenses can be so high, DR so solid, and damage inflicted so terrible that mooks just really waste the mooks, not the PCs. They choke the battlefield and provide bad footing for enemies trying to follow up on the effect of the mooks.
They aren't useless, but a "mook rush" generally isn't a serious threat in GURPS DF. You can maximize what they do, but my experience is that they aren't really that threatening.