Parrying Unarmed Attacks GURPS Basic Set Campaigns p. 376 says:
Parrying Unarmed Attacks: If you successfully parry an unarmed attack (bite, punch, etc.) with a weapon, you may injure your attacker. Immediately roll against your skill with the weapon you used to parry. This roll is at -4 if your attacker used Judo or Karate. If you succeed, your parry struck the attacker’s limb squarely. He gets no defense roll against this! Roll damage normally.
This rule has some problems. It's actually pretty vague about what kind of damage you do (normal damage, duh) and it can result in some silly outcomes and degenerate tactics.
