It has been over two years since the Live with Gold program was announced, and to celebrate, Games with Gold will offer two new games on Xbox One every month starting in July 2015. Every month, Xbox subscribers look forward to seeing what new Games with Gold games will be for free. So far, there is a lot speculation as to what Xbox Live Games with Gold will be free games for September 2015, but there are a lot of games still available for August 2015.
For the Xbox One, there are two titles that include Metal Gear Solid V: Ground Zeroes. The Metal Gear series is a very old video game franchise, and Ground Zeroes is part of the sub-series of prequels of the Metal Gear series, taking place after Metal Gear V: Peace Walker and prologue for Metal Gear Solid V: The Phantom Pain. The game is about Snake as he sneaks into Camp Omega in Cuba to rescue Cipher agent Paz Ortega Andrade and former Sandinista soldier Richardo "Chico" Valenciano Libre.
Also for Xbox One is How to Survive: Storm Warning Edition. According to Gamespot, this is essentially 2014 repackaging of the survival horror action RPG played from a top-down perspective that is about surviving an island that is full of zombies. The difference is that storms will come, and can often put out the fires that you might need for survival.
By the way, a title that was available for the Xbox One last month is also available for this month. So Many Me can be played until August 15th.
The Xbox 360 has two titles which are worth claiming through the Xbox web store, as 360's Games with Gold titles may eventually be playable on Xbox One through backwards compatibility, according to Gamespot.
The first of the Xbox 360 titles is Metro 2033. This one is a Russian game based on a post-apocalyptic science-fiction novel by Dmitry Glukhovsky, and it is set in a world where the atomic bombs have fallen. The survivors took shelter in the underground metro stations, and they have now become independent nations.
It would appear that the Xbox 360 is having the month of Metro as Metro: Last Light is also available. This takes the story of Metro 2033 to the next level with a lot of stealth and survival horror elements.
I'll go ahead and answer the question that everyone is looking for. The answer is sadly no answer, as the free Games with Gold are not usually announced until the month turns, and then there are usually only rumors. After doing several searches for September 2015 predictions Xbox Live Gold and similar keyword phrases, I could find no hint of any possible free game for September 2015. I suppose that I could probably start a rumor with a guess, but it really is anyone's guess.