It has been a while since shoppers minded what Gap was selling -- even before the recession. Problems with the appeal of its merchandise have vexed Gap at a time when its friendly prices and back-to-basics approach should be attracting customers. In a major effort to address that, Gap is reintroducing its core product, denim jeans, with an elaborate campaign notable for its many nontraditional elements, which include a Facebook page, video clips, a realistic online fashion show on a virtual catwalk, and an application for the iPhone.