Seems like it according to this article: http://news.teamxbox.com/xbox/12042/Gears-of-War-Coming-to-Windows-Vista/
Today, ConsoleWars.de reports that a Swiss release scheduled list from Microsoft shows Gears of War as a Windows Vista game coming in the fourth quarter of 2007, following the steps of Halo 2, which is being released exclusively for Windows Vista this winter.