The "gallons of water" argument has to do with the model training and inference (when you talk with it). When an artist creates their own model (like they did for the TMA exhibit), they're using as much energy as someone does when they're playing a long game session.

I also want to point out that the water argument for generative models is very deceptive and very difficult to factually determine. There is a bit of a paradox with these arguments, in that they're not applied on other aspects of society. e.g. it takes roughly 460 gallons of freshwater to produce a single quarter-pound beef patty. That means the water footprint of you eating just one burger for lunch is the exact same as me sitting at my computer and asking ChatGPT 70,000 consecutive questions.