There's a nice dressing room hidden in the back corner, which is where Frisk was before Gaster came in.

They amble out wearing a cool dress. (It's not the first they've tried on, but it is the one that they managed to get on. A lot of this room's dresses have an absurd about of lace in the back.)

And then Frisk pauses.

That feels... like Gaster. But it doesn't look like the one they know.

"Hi?" they try.