I'm going to tell my gf that my aunt tried hooking me up with her friend's daughter
Why are you going to tell your girlfriend that?
Because I plan on having sex with her anyway.
Although it will be hurtful for your girlfriend to hear, it is better that you are honest with her about your intentions. This will enable her to make a choice that is best for her in regards to your relationship.
I don't want her to be in on this plan because I like surprising my gf with sex. I want to be able to take advantage of her without her consent.
You should never have sexual relations with someone that hasn't given you consent. That is a very serious crime.