From now through March 2, 2011, when you buy a pair of pants at the regular price, you can get a top of equal or lesser value for free.
The details are available on the Gap Facebook page.
Here's the fine print: "Does not include jackets, coats, vests, or blazers. Applies to regularly priced merchandise. Second item must be of equal or lesser value. Discount taken at register, and is allocated across items. Not combinable with other offers or discounts. Offer not valid online or at Gap outlet, Gap factory store, The Gap Generation stores, or in Gap 1969 stores located on Robertson Blvd in Los Angeles, or Spring and Broadway in New York. Not valid on Junkfood?. Price adjustments allowed only once within 7 days of previous purchase."