The Freeze Gun was the custom made weapon of choice that was used by Mr. Freeze.
The cold gun first appeared in Batman #121 back in 1959 when he was known by the name of Mr. Zero. His gun was a small yellow canister with two barrels, one released the freeze ray while the other fired blistering heat.
The freeze ray was changed to a much larger two-handed rifle weapon. (This also when he was renamed Mr. Freeze and his skin color was changed to blue)
In the 1992 episode Heart of Ice, Mr. Freeze's gun is reduced to being a small one-handed pistol-like weapon again (vaugely similar in shape to a Mauser C96). Freeze stole parts from his former place of work, Gothcorp. Those parts were needed to construct a large Freeze Gun that was needed to battle his former company of employment. After Freeze was taken into custody by Batman, his suit and weapon were confiscated.
In the 1997 film, Mr. Freeze carried a large freeze rifle with a dial that set the temperature to freezing or thawing. If a victim was frozen, they would only have eleven minutes left to thaw out before they froze to death eternally. In promotional material and the novelisation, Freeze also carried a smaller handgun version of the weapon. He also used the telescope at the Gotham Observatory to make the biggest Freeze Gun of all.
Many years later in the future, Bruce Wayne kept Freeze's gun in a museum in the Batcave. His successor Terry McGinnis who was now wearing the mantle of the bat used the weapon to defeat a dangerous villain named Inque by freezing her solid and shattering her into pieces. The weapon was later destroyed by the same villain when Terry attempted to freeze her again. When Freeze himself returned in one episode, he had Freeze Guns built into the gauntlets of his new suit.
When Professor Hugo Strange had opened Arkham City, Freeze was one of the first inmates who were sent there. Freeze settled into the old GCPD Building where he continued his research in the forensic labs. Freeze was captured by Pengiun, who took his suit and weapons due to the violent gang war that he was in with The Joker and Freeze worked on a cure for the Clown Prince of Crime's Titan Disease.
While Freeze's suit was placed in an exhibit in the Armory of the Museum, Penguin took possession of his Freeze Gun and used it against Batman. However, Freeze gave Batman a method of disabling the weapon in the form of the Disruptor. That gave Batman enough time to disarm Cobblepot and took him down only for him to also take down Solomon Grundy as well. After the fight, Freeze reacquired his Freeze Gun and returned to the old GCPD Building where he worked on a formula for the cure to the Titan Disease that was killing both Joker and Batman.
Batman returned, after he engaged Ra's al Ghul in combat, and gave Freeze the blood sample that was needed to make the cure. After he fabricated the cure, Freeze turned on Batman in the hopes of demanding him to save Nora from the clutches of Joker. Angered by Batman's refusal, Freeze began a fight between himself and the Caped Crusader and used his Freeze Gun in the process.
In the DLC Pack of Batman: Arkham Origins, Freeze utilized an advanced suit with two powerful freeze rays that were built into the arm. Freeze used that to invade Wayne Manor in search of Ferris Boyle whom he managed to find and brought him to GothCorp. Once there, Freeze demanded Boyle give him the codes that were needed to access the lab where Nora was being held, and even went as far as threatened to freeze Boyle so that he would share his wife's fate.
When Batman arrived, Freeze and The Penguin's Henchmen began to combat the Caped Crusader. Freeze was defeated by Batman and was almost killed by Boyle had it not been for the Dark Knight. After that, Freeze's suit and weapon were confiscated after he was sent to Arkham Asylum.