GURPS Zombies 53 is -30%:
- applies only to close-combat grappling
- no effect on Basic Lift
- never benefits carrying, lifting, pulling, pushing, shoving
GURPS Martial Arts: Technical Grappling 28 is -70% for the more specific "(Limb) Grip ST" which:
- applying to only one set or type of limbs
- called Bite ST when it applies to the jaw/teeth, the disadvantage of only having it benefit 1 attack (unless Extra Head, pg 27 gives same) might be offset by the benefit of it also improving bite damage, according to page
Technical Grappling mentions "If more than one set of limbs are impacted, buy Lifting ST instead." Zombies came out December 2013 after Technical Grappling, so the -30% version which benefitted all grappling (but not lifting) hadn't been published yet.