I'm not going to get my gf anything for her birthday
Why wouldn't you get your girlfriend anything?
The reason behind it is that I'm simply a selfish man.
It is good to admit your faults but you should still get them a present. It will complicate your relationship if you don't give her a gift.
Nah, I don't like to give her presents; she gets one from her parents.
But if you are her partner then it's rude for you to not get her something. Even if it's something small, you should always get her a gift.