Chinchen said: She doesn't even need a GUN to play this game.
She can simply just open up a gap to the last stage from stage 1, and open up more gaps all around GLaDOS to let the rockets one by one hit. Then simply just walk out of there when everything starts going bad.
Surely Yukari is GLaDOS, and someone else is Chell? Since who else could make a portal device for testing? If Yukari just gapped everywhere, she could walk out of the area in two gaps. She's made a device that gives people limited control over her powers, and needs to test it.