Art Peck , who has been the chief executive of Gap Inc. since 2015, is leaving the company after 15 years and will be replaced on an interim basis by Robert J. Fisher, its nonexecutive chairman and a son of the couple who founded the retailer.

Gap, which also owns Banana Republic and Old Navy, did not provide any details of the departure or a statement from Mr. Peck in a release on Thursday. The company also cut its outlook for the third quarter and the year, citing slower traffic and issues with its products and operations.

Mr. Peck was tapped for the chief executive role after serving as Gap’s digital chief. He was charged with ushering the major operator of mall stores into the era of digital commerce, as browsing on smartphones and computers replaced visits to stores. But it has been a challenge to revive sales at the company’s flagship brands, especially Gap and Banana Republic, and strike the right balance between its digital properties, full-price stores and outlet locations.

The company’s biggest transformation under Mr. Peck is not yet complete, making his departure surprising. He said this year that Gap would spin off Old Navy into a separate public company in a transaction that is expected to close next year. Gap would retain its existing brands, as well as Athleta, Intermix and its newer Hill City brand.