I'm going to tell my girlfriend that she could just go sleep outside in the cold when she complained about the noise from my fish tanks' filters preventing her from sleeping
Are you mad at your girlfriend because of something else?
I'm furious since I found out the existence of her daughter, and she says to me it's none of my business.
It is okay to feel upset with your girlfriend for keeping secrets, but it is not okay to ask her to sleep outside. It will make you look heartless.
She can sleep in the living room and deal with it. Her daughter has been living on her own for more than 6 years, so my girlfriend can handle it.
Despite everything, you should not take out your anger at your partner. Telling them to sleep outside is cruel, try perhaps buying quieter fish tank filters.