By the book, I don't think there's any way for a character or creature to knock a target prone unless they have a power that specifically does that. D&D 4e--and, by extension, Gamma World--lacks any equivalent to D&D 3e's trip attack.
Disarming is even less common: I'm not aware of
any effect in GW or even D&D 4e that can deprive a target of its weapon (but I put one in
my "Spiked" origin, anyway).
That said, I don't see anything wrong with letting players improvise attacks that do that kind of thing, especially if they rely upon situation or environmental factors (if you let a player knock a monster prone by using a patch of icy ground, they're not going to try doing the same thing in every fight). This is what
terrain powers are all about.
Another idea--if a very homebrewish one--is to let players roll some kind of improvised attack in return for some kind of negative side-effect if they miss (or even if they hit, if they're asking for a really powerful effect). For example, a player might roll 1d20 + Dex + level vs. Reflex to snatch a porker's flail out of his hands . . . but have to grant combat advantage to all adjacent enemies in order to try it, and also trigger an opportunity attack from the target if he misses! Scary risks, but an unarmed porker loses access to his main attack, so it might be worth it.
It'd be up to the player and GM to negotiate what kind of roll the attack would require, and what kind of results there'd be for making or missing it. If the player doesn't like the stakes the GM offers, he can suggest something else, or back out of the wager before rolling.
