The Gunners are a violent mercenary outfit, for whom no job is too brutal to undertake. They are commonly regarded as no better than common raiders, as they will attack anyone they believe is too weak to defend themselves. As long as it improves their bottom line, everyone's a potential target. As a result, while Gunners claim to be soldiers for hire, but throughout the Commonwealth, they have a well-earned reputation as being a highly militarized gang of thugs; with combat training considered equivalent to the Brotherhood of Steel. Aggressive and fiercely territorial, Gunners are guaranteed to attack anyone who wanders into an area they’ve claimed as their own. They’re well equipped and well organized, and a group of Gunners almost always includes at least one combat-ready robot.
Perhaps the most heinous crime the Gunners perpetrated was the Quincy Massacre, when they attacked the southern settlement in force. A desperate defense could not withstand their onslaught and the ruined town fell - with it, the Commonwealth Minutemen. Only twenty people made it out of there alive.
Curiously, the Gunners seem to be following the orders of another party, but to date, nobody has learned who pulls their strings.
↑Fallout 4 dialogue: "Damn Gunners, they're just raiders with fancier weapons. They'll attack anyone they think is too weak to defend themselves."
↑Robert MacCready: "This should send a message to the Gunners to stay off my back." The Sole Survivor: "You aren't afraid the Gunners will retaliate?" Robert MacCready: "The way these lunatics act, you'd think they would... but I know better.For the Gunners, it's always about the bottom line. They just lost this entire waystation and that cost them big.Besides, they have no way of knowing I was involved. Anyway, I guess I owe you a favor now. After all, you hired me but I'm the one that dragged you out here." (Robert MacCready's dialogue (Fallout 4))
↑Fallout 4 Vault Dweller's Survival Guide Collector's Edition p.36: "GUNNERS Gunners claim to be soldiers for hire, but throughout the Commonwealth, they have a well-earned reputation as being a highly militarized gang of thugs. Aggressive and fiercely territorial, Gunners are guaranteed to attack anyone who wanders into an area they’ve claimed as their own. They’re well equipped and well organized, and a group of Gunners almost always includes at least one combat-ready robot. Gunners can be found in Mass Bay Medical Center, Hub City Auto Wreckers, and many other locations throughout the Commonwealth. Gunner Humans Low-ranking Gunners are usually limited to the protection offered by simple clothing, accessories, and the occasional piece of leather armor. Gunners who have reached the rank of Private sport a few pieces of combat armor. Further promotions grant Gunners better-quality pieces of combat armor. Gunners also enjoy a vast arsenal of weapons. Combat rifles and laser rifles are popular choices—and no Gunner would ever be caught relying on a handmade pipe gun—but virtually any projectile weapon weapon has a chance of appearing in a Gunner’s hands. Even valuable plasma weapons can be found on high-level Gunners. Batons and rippers are sometimes used, as are fragmentation grenades, Molotov cocktails, and a variety of heavy guns. Gunner Robots With the exception of custom paint jobs, Gunner robots are identical to their standard counterparts. When fighting a Mr. Gutsy, check its arms to estimate its offensive capabilities, then attack its eyes and torso. It’s often best to slow an Assaultron by crippling its leg. Simply using your most powerful weapons against a Sentry Bot is usually your best option." (Fallout 4 Vault Dweller's Survival Guide Groups, Gangs, and Factions)
↑The Sole Survivor: "What was the Quincy Massacre?" Preston Garvey: "I thought everyone in the Commonwealth knew about that by now.Where the Minutemen betrayed each other, and the people they were supposed to protect.I was with Colonel Hollis's group. A mercenary group called the Gunners was attacking Quincy; the people there called for the Minutemen to help.We were the only ones that came. The other groups... they just turned their backs. On us, and the folks in Quincy.Only a few of us got out alive. Colonel Hollis was dead. So I ended up in charge of the survivors.We never found a safe place to settle. One disaster after another... you saw how it ended, in Concord." (Preston Garvey's dialogue)