No way...I would most definitely go. If you did nothing wrong, there is no need to forgo an opportunity to get dressed up and consume great food and alcohol that is laid on for free.

I wouldn't give a gift either...she will only discover this after you have left. And if they want to whine about you for attending or not giving a gift...then let them, you are in their bad books anyway. Nothing you do or don't do will sway their opinion of you, and the more you shy away or protest...the more they will deflect onto you. Just go, meet some new people, eat nice food...and be sure to swipe some of the silverware.