Here, have some Gaster expressions that will never happen.

Hisssssss

"That's all good and well, but can I go back to bed?"

(Normally, he either wouldn't get up in the first place, or would be wide awake.)

Okay, now this one's not so bad, it's just not an expression he would ever make.

"Gaster. Gaster, Stop. You're scaring the children, Gaster. Please stop."

I honestly don't even know how that one happened.