Public Service Announcement: As of 3/16, Evolve (Xbox One) and Heavy Weapon (Xbox 360, but available on One with Backward Compatibility) are free with Games with Gold if you have an Xbox Live Gold membership. Games with Gold are only free for a month, but if you get them during the month, they are yours for good as long as you maintain your membership.
To buy the games if you are away from your Xbox, you can go to Games with Gold on Xbox.com and queue them for your Xbox One.
