The Gamma Forests was a location of unknown origin. The girls from there were known as "Gamma Girls". The forests there were not known to have ponds, only rivers. It was also neutral from recruitment for the Church. The people there were known to craft handstitched cloth symbols of devotion called a "prayer leaf". They were embroidered with the names of one's children. in the native tongue of the people of the Gamma Forests, as a sign of belief that child would always come home to them.
The first reference to the forests is by Idris as she was dying. She repeatedly told Rory that "The Only Water in the Forest is the River", causing him confusion. Rory recounted the statement to the Doctor, which is dismissed at the time as something yet to come. (DW: The Doctor's Wife)
When Lorna Bucket was a child she once met the Doctor there. She recalled that he repeatedly told her to 'run', implying the situation they faced together had been dangerous. She also implied the reason she enlisted was partly because the Doctor's short visit to the forests was the only thing that had ever happened there. Lorna later crafted and gifted a prayer leaf to Amy Pond, who had recently been seperated from her newborn daughter.
Later as Lorna died, the Doctor pretended to recall their adventure there, despite the fact he did not recall Lorna. It is implied the incident has not yet occured for the Doctor, as he states he always remembers everyone he encounters.
After Lorna's death, River Song appears and mentions the Gamma Forests in part of her argument that some of the horrors he faces now is a result of what he has become. That the kidnapping and use of newborn Melody Pond as a weapon against him is a result of the fear he elicts from his enemies. To hammer the argument home she mentions that the people of the Gamma Forests define "Doctor" as "Mighty Warrior", which makes the Doctor extremely uncomfortable.
Later when the prayer leaf is translated by the TARDIS it shows Amy and Rory that the name Melody Pond becomes River Song, revealing her as their lost daughter. River recounts that the reason the name is translated this was is because "...the only water in the forest is the river." It also implies a closer history between River and the location, due to her knowledge and her affinity to be commonly known as their translation of her birthname. (DW: A Good Man Goes to War)