Gap's shares dropped in after-hours trading after the retailer reported a sales slump that extended into December that included even Old Navy, once a bright spot for the company. The San Francisco-based clothing retailer, which also operates stores under its own brand as well as Banana Republic, said Thursday that a key sales measure fell 5 percent for the five-week period ended Jan. 2. The decline was worse than the 3.5 percent drop analysts surveyed by Thomson Reuters expected, on average.By division, the key revenue measure at Gap fell 2 percent, while Banana Republic posted a 9 percent drop. However, what's more worrisome is that Old Navy, which had been a juggernaut, suffered its second consecutive monthly sales drop. It saw sales fall 7 percent in December, after a 9 percent drop in November.The metric is based on revenue at stores opened at least a year. The company's total revenue dropped 4 percent to $2.01 billion for the five-week period.