Xbox has announced the free games that you will get with your Gold subscription for the month of August. The list can be seen below with the corresponding dates as to when they will be available.
Slime Rancher - August 1-31 (Xbox One)
Trials Fusion - August 16 - September 15 (Xbox One)
Bayonetta - August 1-15 (Xbox 360 & Xbox One)
Red Faction: Armageddon - August 16-3 (Xbox 360 & Xbox One)