rules introduced on page 25 of GURPS Tactical Shooting which allow a person to fire a grappled gun so long as they win a Quick Contest
failure with handguns implies they blocked the trigger, hammer or slide, and the gun doesn't fire
there is a bonus to this contest if trying to fire a long-arm held by the barrel, since there's no direct impeding of the firing mechanisms
even if this contest is won, the foe can still presumably use a Parry to try and make the fired shot MISS