Help with attack modifer (push/throw)
 

I am trying to mimic the effect a bull would cause when they throw someone with their horns, or kick someone with their hind legs.

I have the striker advantage, but how would I mimic the pushing back / launching someone in the air with an attack? Is there a modifer for this? I didn’t see one in the basic book. Or would I roll a throwing skill after dealing dmg on a successful attack to determine if the target is thrown?

Re: Help with attack modifer (push/throw)
 

Well, a charge that knocks someone down is probably a Slam (B371). Kicking or swinging the horns/head to push someone down or away rather than damage them is a Shove (B372). Remember that normal attacks by a bull will be crushing, and thus do knockback (B378) without any special options needing to be chosen (I would not consider a bull's horns to be impaling, as they are fairly blunt).

Re: Help with attack modifer (push/throw)
 

Some adaptation of the Judo Throw rules might be useful.

Re: Help with attack modifer (push/throw)
 

Building on Rupert's post, a shove done with legs instead of arms is defined in GURPS Martial Arts as a "push kick". Are you talking about aiming the direction of knockback as vertical instead of horizontal, like an MK uppercut? Diagonal would probably be more often, 3D hex maps to deal with that would be tricky though.