The M72 rifle is of German design. It uses an electromagnetic field to propel rounds at tremendous speed... and pierce almost any obstacle. Its range, accuracy and stopping power is almost unparalleled.
The Gauss rifle, also called a "coil gun," is a device used to propel a ferromagnetic projectile by accelerating it through a process of magnet induction. The Gauss rifle (and Gauss gun) are named after German mathematician Carl Friedrich Gauss, who formulated mathematical descriptions of the magnetic effect demonstrated by magnetic accelerators.
The M72 Gauss rifle is an extremely powerful and effective weapon, capable of demolishing most foes in only one or two shots. When equipped with this weapon, the Bloody Messtrait is seldom necessary to see the most violent death animations. Its ammunition, 2mm EC, is quite rare however, like the rifle itself, which appears most often at high levels but still remains uncommon.
It has high damage, the best range of the game with the sniper rifle, and a good-sized magazine. It is also capable of knocking down foes, which is useful for stalling enemies using short ranged attacks. A targeted shot to the eyes deals brutal amounts of damage unlike what Frank Horrigan can handle.
The Oil Rig has a few in some of the ammunition crates.
Some guards in the Shi temple have them. They can only be obtained by killing them, meaning one has to risk making the Shi hostile.
Can be found on Dumont in NCR. He has to be looted for it.
Lastly, coming across a random encounter with a cave that has something in them, the cave may be full of very well armed robbers. with a level around 19, the robbers will be equipped with combat armor, and some will be armed with this rifle (less in number compared to other weapons).
The S.T.A.L.K.E.R. series pays homage to the Fallout series with several weapons, while most of them were cut from the finished product the Gauss Gun was included in the main game and its sequel. It bears striking similarities to the M72 Gauss rifle including a very resembling external appearance, extraordinarily high accuracy and damage output as well as the utilization of rare 2mm projectiles as ammunition.