For reasons tied to things like being very tired, I believe I was thinking in
Dungeon Fantasy Roleplaying Game mode in my earlier answer – sorry! In that game, the off-hand situation is spelled out clearly in
Using the Off Hand (
Exploits, p. 36):
You parry with the "off" hand at ‑2 unless using Main-Gauche or an empty hand.
Which is followed by:
If you have Ambidexterity, you may ignore the penalties above!
Not everybody wants the
DFRPG in their
GURPS, but this is the closest to an explicit statement I can find. If we stick to
GURPS, there's at least this, in
Harsh Realism for Unarmed Fighters (
Martial Arts, p. 124):
The Basic Set treats unarmed combat favorably: there's no "off" hand . . .
That's inclusion by oblique reference, but as I said, it's in the rules. If you want even weirder support, check out
Off-Hand Weapon Training (
Martial Arts, p. 50):
You must specialize by skill; any one-handed Melee Weapon, ranged weapon, or Fast-Draw skill qualifies.
That's support by omission: You can't get OHWT for anything but weapon skills . . . unarmed skills specifically aren't Melee Weapon skills. This could be read as "it isn't available," but the intent is definitely "it isn't needed."