Contrary to what many believed, Epic Games does indeed own every right to the Gears of War IP and can do as they please with it, including deciding on who the publisher will be.
Epic's Mark Rein shed some light on the issue when asked by VG247, saying "we own Gears of War. We can do what we want with it." It's obvious that the studio is more than satisfied with their relationship with Microsoft, otherwise they would've jumped ship long ago. After all, Gears of War is one of the largest franchises on the Xbox 360, with each title grossing well over five million copies sold.
"We have a great relationship with Microsoft, and they helped turn the game into a huge success for us, and we are pretty happy with the way things are going"
Epic currently has an unannounced title to be published by EA that is rumoured to be shown off "when the weather's warm."