Gateway on Tuesday said it plans to buy back just over $315 million in its shares owned by Time Warner, closing the book on a tumultuous chapter in its 19-year history as a PC maker. The Poway, Calif., company said it will move to repurchase Series A and Series C preferred stock and up to 2.7 million shares of common stock. It will pay Time Warner $185.6 million in cash and allow its partner up to $130 million in discounts on future payments under revenue-sharing agreements, for a total of $315.6 million. Gateway sought to make the deal now because it would have had to repurchase the stock for a higher price in December, a company representative said.