Situation 1: I attacked my opponent with 3 creatures with power 4 each, and I had Gratuitous Violence out also. He had no blockers. So I thought this would mean he would take 24 damage. He says that I have to assign Gratuitous Violence to one creature only. So the damage will only be 16.
Situation 2: I have Gratuitous Violence out and I attacked my opponent with a 4/4 creature. He blocks with a 5/5. Do we both die or just me?
Apparently because the word used in Gratuitous Violence’s text is “creature”, not “creatureS”, so he said that means its effect has to be assigned to one creature.
Not an uncommon thought from a new player. In a situation like that the card would be worded, 'whenever one or more creatures...' while something worded as 'when a creature...' or 'if a creature...' means each creature will have the effect. Compare the second chapter of the Raven's Warning to Gratuitous Violence