Gaster doesn't look up from his side as Frisk calls out. He's still bent over the numbers, muttering to himself even as they twist and writhe on the screen, even as he lines try to creep their way out before he smacks them forcibly back into place. Maybe he doesn't hear or maybe he just can't keep up long enough to notice. He certainly doesn't even seem to notice the other doors.

Getting any sort of sense of which door leads where probably isn't helped by the noises that are audible from the other side. There's laughter behind one and the sound of a monster dying behind the other and a roar behind the third and someone is crying behind another...

"doesn't matter," Sans says, from just behind Frisk. "we all get to the same place in the end. have you seen my brother?"

He's gone the second they turn around, though. He's gone, and so is Gaster. But the machine in the room beyond is quiet and dark. The Sans on the operating table is no longer struggling. The occasional shuddering twitch keeps him from being entirely motionless.

Just in case Frisk doesn't want to risk trying any of the doors set into the wall, what appears to be a dog door of sorts has appeared in the protective glass of the window.