The Confiscation Field is a Combine device seen several times in the Citadel. A small room with two entrances to be closed by a Force Field and a high ceiling, it is equipped with four weapon strippers that confiscate and destroy unregistered weapons. During the process, any object or being inside the room will not be subject to Earth's gravity.
The first time, Gordon Freeman is stripped from all his weapons save the Gravity Gun, which is transformed into the Dark Energy Field Manipulator. The HEV Suit also undergoes changes, turning into a more powerful suit. Another device is located near the entrance of Breen's office, and is used to take the modified Gravity Gun from Gordon.
At the start of the game, the Gravity Gun is back to normal. When Gordon and Alyx go back into the Citadel, they go through two other Confiscation Fields, first a broken one, then a second, working, that transforms the Gravity Gun again. It later comes back to normal after the dark energy core's stability is restored.