Dust and Saints Row The Third are May 2014's Games with Gold
From Xbox Lives Major Nelson:
Then on May 16th, Xbox Live Gold Members can download Saints Row: The Third (normally US $19.99) for free through May 30th.
The “Games With Gold” program gives Xbox LIVE Gold Members 2 free games to keep each month. A new title will be available the 1st and 16th of each month.