So, given how ranged attacks can be tough enough already, plus this kind of attack comes at a surprise with a force you can't easily resist, it kinda makes sense that this attack doesn't take additional penalties. Same for the ammo-specific versions which actually do a bit of damage (and are resisted solely with Body). So personally I don't think I'd houserule this one, if only to preserve the rule of cool. With Gecko Grip, on the other hand, you'd get to add your +6 bonus on the defense test as far as resisting the called shot is concerned, but not on whether or not you get hit (matters for damage of the ammo-specific called shots and stuff).
So basically, with Gecko Grip: After rolling attack vs defense, throw 6 extra dice in an attempt to reduce the net hits of the attack to 0, to see if the weapon is blasted out of your hands. (Or throw a different color of dice, so it's easier to overview the entire pool for rerolls.)