Bite ST is a limitation applied to Lifting ST which boosts ST only for the purposes of grappling when using the teeth. It is described on page 28 of GURPS Martial Arts: Technical Grappling and is worth the same as "(Limb) Grip ST".
Both would be included in the broader Grip ST limitation in GURPS Zombies.
It does not improve the actual damage from a bite. For that, Striking ST must be increased. The One Attack Only limitation can make that cheaper. Zombies 54 mentions "This is almost always a bite for zombies"
Striking ST (Bite Only) would be worth 2 points per level. Bite ST can be bought at l point per level, so allowing it to benefit damage would invalidate Striking ST (Bite Only). So it only helps with grappling and "Control Points".
The only way it can boost damage is by spending control points, or the free "half the control points as cutting damage" feature on page 14 of Technical Grappling.
This actually makes it a superior approach to boosting damage for those with Sharp Teeth. It makes more sense to buy 2 levels of Bite ST instead of 1 level of Striking ST (Bite Only) because it results in the same damage PLUS control points. The only possible downside here:
- if it isn't Striking ST, this may not make it harder for weak foes to parry
Jaw ST was proposed at http://panoptesv.com/RPGs/Ads-n'-stuff/JawST.html basically being a variant of Arm ST for the mouth, with a -40% No Fine Manipulators discount if the mouth could not manipulate like a hand.