Online retailer Amazon is currently selling the game, which was originally released in summer 2015, for only $16. It normally sells for $30. Alternatively, Walmart has it for $20. Note that these offers are for a physical copy of the game, not a digital one.
Slickdeals, which spotted the deal, reports that retailer Target will match the deal. But as always, you will want to check with your local store to make sure this is the case.
The next game in the series, Gears of War 4, comes out in October for Xbox One and PC. It is a Play Anywhere title, meaning buying it on Xbox One (digitally) gets you a copy of the Windows 10 PC version and vice versa.