The whole thing in GURPS hinges on the Constriction advantage from Basic p43, with the Engulfing enhancement from Powers p45. This lets you crush, strangle, and pin your victim "hands free", and puts pinned victims "inside" you (where they can attack your insides, owch). Other useful traits are
- Brawling (for biting skill and damage)
- Wrestling (for keeping a hold on the little wrigglers)
- Born Biter (for having a bigger mouth to bite with)
- Biting Mastery (if you have Karate instead of Brawling)
- Power Grappling (because using your ST to keep a hold on the little wrigglers is great)
- Extra "one arm" ST (which is pretty fair for using for bite-grapple-crush ST)
If you have Biotech, consider getting Internal DR (costs 1/5th) to armor your inside from sharp food, or an Internal Innate Attack Aura (again, 1/5th cost) that does a little corrosion damage for crazy-dangerous stomach acid. Normal stomach acid probably only does 1 damage every 10 minutes or something, not 1/second... but hey, monsters!
First, bite to grapple (Martial Arts p115, Teeth box) a victim your SM or smaller. Without any other advantages, this takes full normal hit location penalties. [1] This is the grapple required as the first step of Constriction. On his turn he gets a chance to Break Free from your bite/grapple.
For unmodified Constriction (Engulfing) - if you still have him in your grasp, make the ST vs ST/HT contest to crush him. If you succeed, he's Pinned and Engulfed. Ie: you swallowed him. At this point, he doesn't have to roll to hit you with innate attacks, spines, etc. and you get no active defenses, and follows all the usual rules for being pinned.
This does plain crushing damage each turn based on ST vs ST/HT, as per Constriction.
Your victim typically escapes by escaping the Pin as usual, which is more or less "climbing back up your throat".
There are no specific rules for a victim cutting their way out of a grapple, constriction, or engulf, but I'd say if they can do enough cutting or corrosion damage in one attack to "dismember" a hypothetical arm off you (don't forget to use any "arm st" bought for bite/grapple/crushing here), they've cut a hole in you and escape - just like escaping a regular grab by cutting your arm off. And ow, your poor organs.
So, that's "defaults", and a suggestion for cutting your way out. Default really takes at least two turns, not one (Grab, then attempt to crush, subject is pinned and engulfed (ergo swallowed) once they start getting crushed).
For longer processes, add Takes Extra Time to the base Constriction, or onto the Engulfing enhancement. For shorter processes, buy an Extra Attack and limit it to only for the pin/engulf stage.
[1] the Hydra I built on the forums a while ago bought three extra arms and four extra heads and sharp claws AND teeth, so it ends up with five arm-head-bity-clawy-things, game mechanically speaking. I just called them heads of course.
I figure all the extra points spent on that combo despite not having thumbs justifies "a grapple that damages" for the usual half location penalties rather than "a damaging strike that grapples", but it may be more legit to buy one Extra Attack with limitations "only for adding a grapple to a bite". Or just live with hit location penalties :D
If you want to have an improved grapple-thing and skip over manipulative ability, buy the Extra Arms with Nuisance Effect: Temporary Disadvantage Ham Fisted 2 or so to make sure they're not very good at fine detail work while still having a strong grip.