I want to fart on my girlfriend
Is there a reason that you want to fart on her?
I love my girlfriend and I would like to do that on her.
Almost everyone, your girlfriend included, would find that act to be disgusting rather than funny or affectionate.  You shouldn't fart on people because it will upset them.  Show affection to your girlfriend in a more respectful manner.
I know how to make her happy and i will do it if I want to.
I think that that is gross. Farting on them is disrespectful and you should want to respect your partner more.