One Hand

One Hand (alternatively describable as Missing Hand in respect to other disadvantages) B147, you can still parry with it, unlike One Arm:

you may make unarmed parries with a handless arm

GURPS Martial Arts pg 116 the top-right The Sound of One Hand Grappling discusses a penalty for grappling using the crook of the elbow instead of a hand "because the hand isn't free". This presumably also applies "because the hand isn't there".

Also pg 116 on the lower-right:

while a handless arm can’t punch or grab, it can deliver an Elbow Drop or Elbow Strike.

• Note that grab and grapple are different concepts.
• An elbow delivered to the front works like a Punch except presumably Hurting Yourself applies to the arm instead of hand, like how Shin Kick applies it to the leg instead of the foot.
  • unclear if MA81's Uppercut could also be used to deliver upward elbow strikes. Doesn't seem unbalancing.
  • Two-Handed Punch talks a lot about knitting fingers so it's probably off the table here.
• not addressed is Hammer Fist. Handless creatures do not have fists, but MA73 notes "includes both the true hammer fist and similar attacks". Two ("punches with the palm or heel") are clearly off the table, but one ("forearm smashes") doesn't seem like it would require a hand, and would be an option (like elbow strikes and handless arm parries)
  • forearms would have the advantage of better reach (elbow strike has a range limit for All-Out Attack (Long) which probably applies when using the punch technique) and not delivering an attack with Joints
    • they should in theory have slightly less reach than a heel strike / palm strike / hammer fist (just as those 3 in turn should have less reach than a classic punch) but this is not mechanically represented. It might be useful to distinguish them in Matter of Inches

see also

• Missing Digit:
  • Missing Finger
  • Missing Thumb
• Missing Arm