kill or harm by exposure to poisonous gas.
But it's only at the end of the film that we learn what appears to have really happened: it is he himself who killed his mother, gassing her whilst apparently suffering from a delusion that she is another person.
fill the tank of (an engine or motor vehicle) with gasoline.
after gassing up the car, he went into the restaurant
talk, especially excessively, idly, or boastfully.
I thought you'd never stop gassing