Microsoft just declared which games will be free for Xbox Live Gold members for August 2017.
On Xbox One, Xbox Live Gold members can download Slime Rancher ($19.99 for free during the month of August. Trials Fusion ($19.99) will be available as a free download from August 16th to September 15th.
On Xbox 360, starting August 1st, Bayonetta ($19.99) will be free for Xbox Live Gold members through August 15th. Then on August 16th, Xbox Live Gold Members can download Red Faction: Armageddon ($29.99) for free through August 31st.