I feel like grossing out my girlfriend to stop her from doing something
What kind of things does your gf finds gross?
Not peeing and putting bum snot on my hand and trying to touch her face
You should not do unsanitary things to your partner. It is unkind and gross. She will not appreciate that, and she may be very upset by it.
I'm happy she is grossed out by that. She needs to learn how to stop doing it.
So she is doing the same thing. Basically you're both grossing each other out. If you are having fun with that, then there is nothing wrong with it.