Zero Point Energy Field Manipulator

The Zero Point Energy Field Manipulator (more commonly known as the Gravity Gun) is a tractor beam-type weapon that was originally designed for heavy lifting and the handling of hazardous materials. Introduced to the player by Alyx Vance shortly after arrival at Black Mesa East, the Gravity Gun soon becomes an invaluable tool in Gordon's arsenal.

Usage
The gun has two basic modes, the first (primary fire button) punts objects. This is useful for clearing out barriers and moving heavy objects. The secondary mode (secondary fire button) picks objects up and holds them in its field. These objects can then either be dropped (by pressing the secondary fire button a second time) or launched (by pressing primary fire). In combat, the Gravity Gun can be very useful depending on the types of physics objects available for use as ammunition. The gun can also be used to pull objects over from a distance, allowing you to access supplies or other objects that would, otherwise, be out of reach. It's also worth noting that while the gun can manipulate many objects, some objects are too heavy or bulky to be picked up. However, some of these objects will still move when punted.

Besides its obvious use as a weapon through its ability to hurl heavy objects at enemies, it is also utilized in many other ways. Clearing heavy obstructions, holding up objects as portable shields against gunfire, returning live grenades to their owners, knocking over turrets, removing and reprogramming hopper mines, removing rollermines from vehicles, dragging objects over from a distance, etc.

Finally, while the standard Gravity Gun can't pick up organic material, it can be used to effect against some enemies. Manhacks, City Scanners and Shield Scanners can all be held in the gun's field and launched. The gun can also punt Headcrabs and Antlions, inflicting minor damage. Doing so also knocks the latter over, exposing their weaker undersides.

The inner workings of the gun is unknown. However, it is possible that both firing modes are powered by a micro black hole, held in check by a containment system with some Combine technology. The secondary fire might be achieved by a controlled power-down of the containment field. In light of dark fusion and tunneling entanglement devices (which create an artificial singularity), it is strongly possible that Combine technology can harness the power of black holes. It would also explain the increased power of the gun when supercharged (see below): the energy surge from the confiscator have shorted out the containment system, unleashing the power of the micro black hole while the gun is fed on residual dark energy.

Supercharged Gravity Gun
Also known as the "Dark Energy" Gravity Gun.

In Citadel security systems, the Gravity Gun seems to simply absorb the weapon confiscator's energy, while other weapons are destroyed. In doing so it becomes 'supercharged', and shorts the weapon confiscator out in the process. Once supercharged, the Gravity Gun emits blue light rather than orange, it quakes in Gordon's hands and creates small bolts of energy across it's muzzle, and is vastly strengthened. It can hold larger objects, launch them at greater speed, pull with more force and even pick up and launch living creatures (specifically Overwatch Soldiers) to their deaths. Also, holding someone in the supercharged gun's field appears to electrocute them, as the body will spark and twitch, once dropped.

The Gravity Gun is supercharged for the first time in Half-Life 2's 12th Chapter, Our Benefactors, when Gordon's pod deposits him into a confiscation field and remains so until the end of the game. During this time, Gordon's HEV Suit also becomes 'supercharged' to the point of harboring 200% energy, and power chargers being capable of replenishings Gordon's Health. At the start of Episode One, it is no longer supercharged, but becomes charged again after Gordon and Alyx reenter the Citadel to stabilize the core and Gordon accidentally enters another confiscation field. It remains supercharged until Gordon reactivates the core's containment field, at which point the Gravity Gun reverts to its normal state.

After leaving the Citadel, the Gravity Gun does not become supercharged again. Valve has confirmed that, sadly, the Gravity Gun will never become supercharged again due to the difficulty of incorporating it into the gameplay of the level.

In Episode 1 and 2 it can be activated in normal use by entering the console command "physcannon_mega_enabled 1".