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Gravity is the attractive force acting between two objects of mass. The most common and representative gravity that occurs in nature is the gravity of planetary bodies. This causes everything on such a body to move towards their center of mass. Naturally, Earth also has gravity, and the Moon and Xen have a weaker gravity than Earth.
Some technology in the Half-Life and Portal universe can manipulate gravity to control in-game objects. For example, the Gravity Gun and Gravity Gloves use zero-point energy fields to manipulate objects at a distance. The Portal Gun has a weaker version of the technology. It is able to levitate relatively small objects at a small distance in front of it, but cannot attract objects from far away.
Combine Advisor used psychokinesis to restraint Alyx and Gordon. (Half-Life 2: Episode Two)