Built on a sloop facing east, Gasa Dzong is the administrative headquarter of Gasa Dzongkhag, which consists of the Mochhu valley locally known as Goen Khartoed in the upper region and Goen Kharmed in the lower region and extends as far as the Punakha Dzongkhag. The locals say that the entire landscape has the appearance of the image of Tseheringma, Goddess of long life; and the Dzong, named Trashi Thongmon, is built on the chest of this image, which happens to be the local deity of Gasa.

Legend has it that when Zhabdrung was making his way to Bhutan, the deity of Gasa traveled to Tsari Kibuthang in Tibet to greet him. The Zhabdrung blessed the deity with the new name of “Trashi Thongmoen”. Thus, the Dzong was also given the same name in 1648, since the deity was its protector.
 
The local residents say that Terkhungpa, who was skilled in iron works, lived and worked in the area. Thus the name of the area is called Garsa, which literally means ‘the land of black Smith’. The Dzong is also known as Garsa Dzong. The name is suitable as it is also said that the principal occupations of the people in the early days was blacksmithing. It takes nine hours on foot to arrive at the famous hot spring, which lies at the bottom of the slope after leaving the last road head at Trashithang.

Gasa Dzong has a circular shape with three towers at strategic points, which proves that it was a defense Dzong. The utse is built directly above the cave where Terkhungpa supposedly meditated. The temple of Kap-Goenm, or Kazhimai Lhakhang, lies to the left of the cave, which houses the images of Buddhas of the tree eras, the past, the present, and the future; the Zhabdrung Ngawang Namgyal; and Guru Rinpoche. The room is also used as a meeting hall especially during the Dzongkhag Assembly sessions. Attached to the temple with but different access is the Chag Dzoed Lhakhang that was used by Zhabdrung as his residence when he visited the Dzong after its completion. It has an altar, a skeleton of the sheep that actually came from Tibet with Zhabdrung, and a riding saddle. But the words Chag Dzoed also indicated that it could have been a private store of the Zhabdrung, not just a residence.

Directly below the cave is the Zimchung sarp, the new residence, which now houses the Dzongkhag Thrimkhang (District Court). The Dzongkhag administration is situated at the right side of the cave. In the past this was the residence of the Gaza Dzongpon. The utse is a three-storied building. Its first floor is the Lama Lhakhang, which includes all the Kagyu Lamas from Dorje Chang (Buddha Vajra Dhara) to Mailtrya, the future Buddha.

The ruined structures of the three Ta Dzongs were suggest the Dzong was used as a defense fortress during the various Tibetan invasions. The central Ta Dzong is located at the left side of the utse, while the other two were located about fifty meters to the left and right side of the Ta Dzong. Apparently, the three Ta Dzongs resembled the tormas, ritual cakes, of Yeshey Gonpo (Mahakala), the protective deity of Bhutan. The inside of the Ta Dzong was apparently used to garrison the armies who would also keep watch over the paths from Lingzhi on the northwest side and from Laya on the northeast side, through the narrow holes drilled for this purpose.

A contingent of the monastic body led by the Drapai Lopon (head of the lexicography division of the monastic body) visit Gasa to make the annual offerings to the deities in the last week of the 10th month of the Bhutanese calendar every year. In honor of Mahakala, gun salutations are performed on the 29th day. It is a common belief that during the time of Terkhungpa there lived a famous Bon master in Gasa. During a competition of spiritual attainment, the Bon master also proved that he could walk up the cliff with ease. Terkhungpa perhaps expressed his appreciation of this great achievement. Today, as a token of appreciation to the Bon master, a Bon priest must also participate in the ceremonial procession of the offerings to the Mahakala, wearing his full ritual attire. Gasa Tshechu is held annually at the same time as Thimphu Tshechu. However, the first day of the tshechu is held in Phu Lhakhang, the original place of the festival.