Cap Gemini plans to buy United States-based IGATE Corp for $4 billion cash to make North America its biggest market and is raising its sales outlook for 2015 after a solid first-quarter, the French IT services company said on Monday. IGATE, a technology and services company with strength in the financial services sector but also active in retail, manufacturing and healthcare, had 2014 revenue of $1.3 billion, with double-digit growth and a 19 percent operating margin.