For the month of May, Xbox Live Gold members will receive four new free games – two on Xbox One and two on Xbox 360 – as part of the Games with Gold program. You can play both Xbox 360 titles on your Xbox One with Backward Compatibility.
On Xbox One, Xbox Live Gold members can download Defense Grid 2 ($14.99 ERP) for free during the month of May. Costume Quest 2 ($14.99 ERP) will be available as a free download from May 16th to June 15th.
On Xbox 360, starting Sunday, 01 May, GRID 2 ($24.99 ERP) will be free for Xbox Live Gold members through May 15th. Then on May 16th, Xbox Live Gold Members can download Peggle ($9.99 ERP) for free through May 31st.
Source: Major Nelson