The Gravity Gun has two basic modes. The primary fire releases an energy blast, which pushes the targeted object with a tremendous force. This is useful in-game for clearing out barriers, or moving objects that cannot be picked up, such as disabled cars. The secondary fire pulls objects towards the gun, and holds them just ahead of the device. These objects can then either be dropped by pressing the secondary fire button a second time, or they can be launched forward with the primary fire. The secondary fire can only pick up lighter objects — mainly those which Gordon himself can carry, with some exceptions — but the primary fire is capable of pushing much larger objects even if the secondary fire cannot affect them. The gun cannot affect most organic material, though there are a few exceptions.

Using the Gravity Gun's secondary and primary fire modes in conjunction allow it to be used as a powerful weapon. Since it uses physics objects rather than conventional ammunition, there is virtually no limit to how many times it can be fired. However, this advantage is balanced by the need to "reload" after every shot, by grabbing a new object. Depending on the object fired, the Gravity Gun can be one of the most damaging weapons in the game. Heavier objects like metal barrels and propane tanks can even kill an Overwatch Soldiers   with a single blast, but conversely, objects such as plastic crates, plastic bottles, or wooden pallets will do very little to no damage due to their weaker materials. Explosive barrels, in particular, are highly devastating when used with the Gravity Gun, but this can backfire if the barrel is shot by an enemy while being held by the gun. The Gravity Gun is most effectively used as a short to mid-range weapon, since the physics objects it uses for ammunition are affected by gravity once they leave the device's hold

The secondary fire of the Gravity Gun grows weaker with distance. Outside of a certain range, it is able to pull objects closer, but much more slowly and the object is still subject to Earth's gravity. The gun must also be constantly trained on the object to maintain the pull until it has been captured

In addition to its primary uses, the Gravity Gun can be used in more esoteric ways  Shield Scanner, Manhacks and City Scanner can all be held and punted by the gun; the first, in particular, can be held by the gun for use as a makeshift chainsaw. All three are heavy enough to deal significant damage if pushed, and will be destroyed if pushed into a nearby wall. However, if pushed over greater distances, they will stabilize their flight. It can also be used to grab and recalibrate Hopper mines  or remove Rollermines  from vehicles. A Rollermine grabbed in such a manner can be reprogrammed by Alyx to attack hostile targets instead of friendlies, and can also be pushed into enemies directly. Combine Sentry Gun aka turret can also be manipulated by the gun (disabled by being thrown when being enemies, replaced after being knocked down when being friendly).

In addition to its use as a weapon, the secondary fire of the gun can be used to pull objects over from a distance, allowing Gordon to access supplies or other objects, that are out of reach or behind a force field

The Gravity Gun is also handy for moving trapped vehicles, such as Muscle Car (the junker), the , as well as the Scout Car (the buggy) . This is particularly efficient for flipping the Scout Car, if it flips over onto its roof

Another useful strategy is throwing back a Frag Grenade to its sender. This can be dangerous, however, as the grenade can detonate while the player is holding it

Heavy objects, such as disabled cars, or even some pieces of furniture can only be pushed using the primary fire. Cars can kill several enemies at once and are also useful in Episode One, when Freeman must block several Antlion holes with them

The Gravity Gun can also be used to grab larger objects, such as metallic plates or radiators, and holding them up as makeshift shields. This was demonstrated by Valve in the E3 2003 demonstration Traptown.

When exposed to the Confiscation Field in the Citadel, the Gravity Gun is "supercharged", rather than being vaporized as Gordon's other weapons are. It also cripples the field itself in the process. Once supercharged, the Gravity Gun emits blue light rather than orange, quakes in Gordon's hands, and creates small bolts of energy across its muzzle. Its powers are vastly enhanced, enabling it to manipulate objects of far greater mass than it could normally (such as ), as well as pull and repel them with greater force.

Its most significant enhancement, however, is its ability to manipulate living, organic matter. Any Overwatch unit pushed by its primary fire is instantly killed, and their body retains an energetic charge, that will kill others if it strikes them. If grabbed by the gun's secondary fire, they are likewise killed instantly.

It also appears to have the side effect of vaporizing any weapon the soldiers are carrying, even when they are killed indirectly by a thrown object. However, this also occurs when kills an enemy herself using her handgun, as well as when the soldiers are killed by their own grenades or other causes, suggesting a Citadel-wide security measure destroying their weapons rather than a Gravity Gun induced effect.