Well since you have that condition already, there is nothing to do but face it. Show them what you are, if they look at your good side and not in your condition then they are good people who knows how to deal with people no matter how they look. And despite that, your girlfriend still would want you to meet her parents or family, that is a good sign that she is proud of you. I would suggest you to tell them about your condition because if you will try to avoid looking at them in the eye, then they would think you are not a sincere person or something to that effect. If they dislike you, face it too. Life has to go on. Continue to be what you are.