Rockville’s Congressional Plaza, once known for its upscale boutiques and specialty shops, is increasingly becoming a hub for off-price retailers. The latest addition: a Gap Factory store, opening this summer in the former Lands’ End space. Gap Factory, which sells lower-priced versions of Gap’s classic styles, will be right at home in a lineup that now includes a J. Crew Factory store, a Roche Bobois furniture outlet, and the Dick’s Sporting Goods clearance store Going, Going Gone! Later in the year, Congressional also will welcome a new Nordstrom Rack. Gap still operates a full-priced store at Pike & Rose, but it closed the one at Westfield Montgomery mall last summer.