The Gamma Quadrant was the common designation for one-quarter of the Milky Way Galaxy. It was adjacent to the Alpha Quadrant and the Delta Quadrant. One-quarter of the galactic core was located in this quadrant.
A Barzan probe sent through the Barzan wormhole discovered that the Gamma Quadrant terminus was located beyond the Denkiri Arm, a distance that at Warp 9 would have taken nearly a century to cover, this later shifted to the Delta Quadrant and the wormhole was discovered to be unstable.
In 2369, the Bajoran wormhole in the Bajor system was discovered. This wormhole had a terminus in the Gamma Quadrant that was four light years from the Idran system. The wormhole linked this quadrant with the Alpha Quadrant.
A Tosk came through the Bajoran Wormhole from the Gamma Quadrant and docked at Deep Space 9 on a hunt, and was pursed by Hunters. The Hunters said that Tosk was bred to be hunted by them and had spent his entire life preparing. Later, the Hunters were allowed to resume the hunt and take the Tosk back with them to the Gamma Quadrant, after Chief O'Brien helped the Tosk to escape and leave back through the Bajoran Wormhole and into the Gamma Quadrant.
Deep Space Nine had the distinction of making first contact with a Gamma Quadrant species known as the Wadi. However, the Wadi showed no interest in formalities and headed straight to Quark's. The Ferengi initially saw the Wadi as a potential source of great profit, but when the Wadi Master Surchid Falow started to win at Dabo and went on a winning streak, Quark had one of his waiters rig the game. The Wadi caught him and forced Quark to play "an honest game" called Chula, using living people as players, but this was revealed later to be only a game and not really dangerous.
Kai Opaka was taken through the Bajoran Wormhole by Benjamin Sisko, where they received a distress signal. They then found a planet with a network of satellites, one of which was malfunctioning. Their runabout approached the planet and one of the satellites attacked it, forcing them to crash land. Opaka died in the landing, which devastated Kira; however, her mourning was interrupted by a group of people called the Ennis are who were at war with a group called the Nol-Ennis, both groups were being regenerated by some form of extraterrestrial nanotechnology. The same technology revived the Ennis and Nol-Ennis, but rather than a blessing it was a curse: both groups were sent to the planet as punishment for centuries of conflict, doomed to die over and over, Kai Opaka accepted that she could not leave the planet in the Gamma Quadrant and felt called to help the two warring factions learn how to live in peace.
The Vulcans were at the forefront of the exploration of the Quadrant, beginning only months after the wormhole was discovered by Benjamin Sisko and Jadzia Dax in 2369. The Vulcans made many discoveries, including the remains of the Hur'q race.
The Klingons showed an interest in exploring the Gamma Quadrant shortly after its discovery by sending their own scout ships to the quadrant. The IKS Toh'Kaht was among the first ships to explore the quadrant in an effort to establish colonies on the other side of the wormhole. They also operated freighters to the quadrant.
The planet Meridian was also located in the Gamma Quadrant, and spent most of its time in another phase of existence, where its people existed only as consciousness, with no physical being. The planet was in this dimension for a very short time, spending twelve days here, then it disappearing for sixty years. Each time it returned, the planet would have less time in this universe, and eventually would stop appearing.
The starship Defiant was caught in a type of "quantum-bubble" surrounding an inhabited planet. After investigation, it is discovered that the inhabitants are the descendants of the Defiant's crew. Generations of descendants have been living on the planet for many years and their own history began when the Defiant tried to escape from the bubble. Instead of passing through its energy barrier, the ship was thrown back in time two centuries and crashed with no hope of contacting the Federation or repairing the ship. Faced with that reality, the small stranded crew of the Defiant (including Dax, Sisko, O'Brien, etc.) decided to remain and establish a society.
The Dominion was established approximately 10,000 years prior to 2375. This galactic power was located between 40,000 and 50,000 light years from the galactic core in an arm of the galaxy. Other powers have been encountered, but their status with the Dominion was unclear.
In the act directly precipitous to the Dominion War, the Federation mined the entrance to the wormhole (leading to the Gamma Quadrant) in 2373. This cut off access to this quadrant from any Alpha Quadrant powers and prevented Dominion reinforcements from adding to the Dominion War.