Garry's Mod or GMod is a modification for the Source game engine. It is a nonlinear physics and role playing video game that allows the player to manipulate objects and experiment with physics.
Garry's Mod was created in 2004 by Garry Newman's Team of Developers. It started out as a mod that allowed users to rope things to go together, and it made the pistol shoot out manhacks and other things to build. Since then, the game has gone through various versions, adding more and more features until it became the game based mod it is today. After the tenth version of Garry's Mod, the mod was no longer referred by its number and just called Garry's Mod.
Garry's Mod allows players to manipulate props from any Source engine game installed on their computer (or ones created by the community) using the Physics Gun, or Physgun, a blue version of Half-Life 2's Gravity Gun (the same model as the Super Gravity Gun at the end of Half Life 2 as well as some parts of Half-Life 2: Episode One) that allows objects to be picked up from any distance, adjusted in mid-air and frozen in place. A second, multi-purpose gun called the Tool Gun can perform many tasks such as weld objects together or rope them together with ropes or controllable winches, plus hanging lights and lamps, doors and buttons, or whatever the community is able to code it to perform.
Person1: "Hey Person2, what are you playing?"
Person2: "Garry's Mod, the best physics game ever!"
Person1: "Wow! I love you!"