Microsoft has announced its July offerings for the Games with Gold program, and with it comes a slight change. Starting next month, Gold members will get two Xbox One games per month instead of one.
Throughout the whole month of July, you can get Assassin's Creed: Black Flag. From July 16 to August 15, you'll also be able to get So Many Me. For Xbox 360, you can pick up Plants vs Zombies from July 1-15, then get Gears of War 3 between July 16-31.
With the incoming backwards compatibility support for Xbox One, having a digital 360 game in your library should make it accessible on your XBO once the update comes. So, adding PvZ and Gears to your library is likely a sound decision even if you don't have a 360 hooked up at the moment.