The game in question is Orcs Must Die which originally came out on the Xbox 360 console back in 2011 and is now playable on both Xbox One and Xbox Series X console generations via backwards compatibility.
It seems as if this random addition to Games With Gold is available in the US and UK though other countries, such as Australia, are just seeing it as being for sale like usual. You can check to see if Orcs Must Die is free for you by checking out its product listing here on the Xbox website.
Is Orcs Must Die showing up as free for you? If so, what region are you in? Let us know in the comments below.