The Gamma Quadrant was the common designation for one-quarter of the Milky Way Galaxy. This quadrant was adjacent to the Alpha Quadrant and to the Delta Quadrant. One-quarter of the galactic core was located in this quadrant. The Denkiri Arm was located in this quadrant (The Denkiri Arm was a spiral extension of the Milky Way Galaxy).
In 2369, the Bajoran wormhole in the Bajoran system was discovered. This wormhole had a terminus in the Gamma Quadrant that was four light years from the Idran system. This wormhole linked this quadrant with the Alpha Quadrant.
The archaeologist Vash explored the Gamma Quadrant for two years at the invitation of Q in 2367. She was found there in 2369 by members of Deep Space 9's crew.
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.
On the other hand, no Romulan ship had ever been to the Gamma Quadrant until late 2371 during the Tal Shiar's abortive attack on the Founders' homeworld.
The Ferengi saw the Gamma Quadrant as an enormous chance to make a profit and their reputations would be, as Gral put it, "absolutely stainless".
The Bajorans also colonized a planet in the Gamma Quadrant, New Bajor, in late 2370. Unfortunately, the planet was invaded by the Jem'Hadar, who massacred the population.
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.