It doesn't have to be a violent death. Imagine you want him to die because of a deadly virus released in a containment area or something like that.
ah..in that case. Make the gargantua go to that certain location (with script)
Place a trigger_once (with the only monster properties) in that location which gargantua will enter. So when he passes the trigger_once, the script will be activated and gargantua will walk to the script. Now place a monster_generic in that same location with the gargantua model (prisoner and not solid enabled) with render fx texture and fx amount 0. After that, make the same script's (which we used for gargantua to walk into it) target to a multi_manager which will activate a trigger_relay (with the kill target set to gargantua's name so that the gargantua will vanish instantly) and an env_render which will bring our monster_generic visible again (render fx: normal fx amount:0), also this mm needs to trigger another script which is set to this monster_generic (our fake gargantua) playing the dead animation.(Important: tick leave corpse) (dont forget to set the angles).
Moar realism: add some fake explosions and a sound playing the gar_die1.wav which are triggered by the mm.
And thats it, we got a dead gargantua lying on the floor.
If still confused. I can make an example map. It's easy.