This game is not what you usually see on Steam. In Garry's Mod, you enter a physics sandbox which provides you with tools to make what you want happen and leaves you to it. You can play alone or join thousands of other players kidding around and playing with physics. If you do try this game, do check out its Steam Community Workshop

Garry's Mod or GMod (stylized as garry's mod) is a sandbox physics game using a modified Source engine, created by Garry Newman. Garry's Mod was originally a modification, a "mod", for Valve's Half-Life 2, but was later made into a standalone game. I ...   more on Wikipedia

XCOM: Enemy Unknown is an action strategy game developed by Firaxis Games. Play as the commander to a special paramilitary unit called XCOM. Your job is to protect earth from global alien invasion. Manage resources, advance technologies, and plan out individual units as well as global scale strategies to fend off the invading foe! Engage in over 70 unique missions, interact and negotiate with world governments, research alien devices and customize your own instruments of death! Build a base, develop it, grow your own units and participate in turn-based close combat!

XCOM: Enemy Unknown is a turn-based tactical video game, developed by Firaxis Games and published by 2K Games. The game is a "reimagined" remake of the 1994 cult classic strategy game UFO: Enemy Unknown (also known as X-COM: UFO Defense) and a reboot ...   more on Wikipedia