Gorz's summon happens during the resolution of his trigger effect, wich activates in the player's hand when battle damage is inflicted.
Any card (spell speed 2 or higher) capable of negating the activation of a monster effect can be chained to it, as long as that card can be activated during damage step.
Examples are Solemn Warning (negates an effect that special summons a monster), Herald of Orange Light, War Chariot, Divine Wrath, Doomcaliber etc.
Sidenote:
Any Counter Trap can be activated in damage step, and any monster effect capable of negating another card effect can also be activated in damage step.