The Legendary Gunsmith was a renowned weapons expert. His skills and reputation netted him at least two apprentices; one of whom worked as a soldier in the CFA and the other who was taken prisoner by the Soviets operating in Afghanistan. At some point during the 1980s, the Legendary Gunsmith was also taken prisoner by Soviet troops and transferred to Yakho Oobo Supply Outpost in Afghanistan.
During their operations in 1984, Diamond Dogs eventually learned of the Gunsmith's reputation and decided that his skills would be a valuable asset to their organization. Acting on intel Diamond Dogs had gathered, Venom Snake went to extract the Gunsmith and recruit him. However, Diamond Dogs' first two reports on the location of the Gunsmith were unsuccessful and only on the third attempt did Snake successfully locate and extract the Legendary Gunsmith.
Behind the scenes
The Legendary Gunsmith is a recruitable Mother Base Staff member in Metal Gear Solid V: The Phantom Pain. Though his title is always the Legendary Gunsmith, his code name under Diamond Dogs is Gray Mongoose.
Recruiting the Legendary Gunsmith grants a player, the ability to completely customize almost every weapon they have developed.