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 also has the ability to knock down foes, which is useful for stalling enemies using short ranged attacks. A targeted shot to the eyes deals brutal amounts of damage, not even Frank Horrigan is able to stand such punishment.
The Oil Rig has a few in some of the ammunition crates.
Some guards in the Shi temple have it. It can only be obtained by killing them, but note that this will turn all Shi hostile towards you.
You can find this weapon on Dumont in NCR. You cannot pickpocket it, therefore you will have to kill him to obtain it.
Lastly, if you come across a random encounter with a cave that has something in them, the cave may be full of very well armed robbers. If your level is 19 or higher, 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.