Neji probably has enough speed to bypass Gaara's auto defense, but his attacks just aren't the right type to hit through his defense. It takes high physical strength or a piercing jutsu to break through it, and Neji's jyuken doesn't fall in either of those categories. He could defend against his attacks, but kaiten isn't going to last forever. It isn't really going to protect him against a mid air coffin either, since you can't spin in mid air, especially in such a tight space.