What do you mean gas? GLaDOS killed one science team, but they installed a morality core. They wouldn't have installed the morality core if they had no intention of using the facility again. So they put more scientists in the facility, but during Portal, there are no scientists. At all. So something must have happened to them.
No shit sherlock. But obviously the Scientists are mother *crag*ing dead, otherwise they would still *crag*ing be there. So what is the point of elaborating on an answer you will never find?