Do you think I should tell my gf about me being an otaku..a hardcore one to add....
Yes, why not? If she likes you for who you are it should not be a problem, and even if she doesn't like it it's better to tell than having to hide it forever.
Does she have any biases towards otaku culture? If so, let it in slowly. Otherwise, just have her sit down and watch some gateway animes with you, and when the subject comes up, spill the beans. You can't make someone else an otaku though. It either grips someone or it doesn't, and trying to force someone to like something you like is a violation of their free will to like whatever they happen to like.
Why is this a dilemma to begin with. If you make doubts, something is bound to happen