Can Gears of War 4 be played offline?
According to Xbox GoW 4 FAQ, you need to connect to your Xbox Live account only the first time that you start up the game. From the site:
You’ll need to sign in to the Xbox Live service via the Xbox app when you launch Gears of War 4. Once successfully connected, you can continue to play offline in Campaign Solo mode only. Campaign Co-Op, Multiplayer, and Horde will not be available until you go online.