А чем не устраивает базовая механика surprise в GURPS?

You also get no active defense if
you’re unaware of the attack.
Examples of situations in which no
active defense is possible include a
stab in the back from a “friend,” a sur-
prise sniper’s shot, and a totally unex-
pected booby trap. And you get no
active defense if you are unconscious,
immobilized, or otherwise unable to
react.

When the PCs surprise a group of
adversaries, or vice versa, the sur-
prised party may not be able to react
immediately. In this case, the attack-
ers should get one or more “free
turns.”