Gyatso was known for his kindness and sense of humor. Though Aang learned airbending under him as part of his Avatar training, the monk always allowed time for fun and games, such as Pai Sho. In his earlier years, Gyatso also had a strong friendship with Aang's previous incarnation, Roku.
Gyatso grew up in the Southern Air Temple. When the sixteen-year-old Avatar Roku traveled to the temple to commence his airbending training, he became close friends with Gyatso. Roku later told Aang that "some friendships are so strong, they can even transcend lifetimes", when his young successor was surprised to see that Gyatso had been both Roku's friend and Aang's. Before starting their gliding lesson, Gyatso and Roku were goofing off while standing in line. During the lesson, Roku showed off a new glider trick by making several loops in the air. Gyatso tried to top his skill by air surfing atop his glider. The young monk soon lost control, but he was saved by Roku. The two-person load proved too much for him to handle, and the two of them crashed into a group of student airbenders on the land below. They showed no signs of injury, however.
Eventually, Gyatso became a master airbender and was promoted to one of the five head monks at the Southern Air Temple. Monk Gyatso was highly respected, a great baker, kind, and very wise, but he remained a bit of a prankster. At some point, Gyatso befriended a younger Guru Pathik.
Twelve years after Roku's death, Gyatso was present with the rest of the Council of Elders when Aang's identity as the new Avatar was revealed. Since Gyatso was Aang's personal teacher and guardian, it also became his duty to better the Avatar's airbending skills. The bond between the two became very strong and intimate to the point where Gyatso became a paternal figure to Aang, as well as a close friend and confidant. He helped Aang with his airbending training, played games with him, and made Aang's difficult time at the temple more enjoyable.
Even though Aang's status was announced as the Avatar, Gyatso believed that he should still grow up as a normal boy. He refused to accelerate Aang's airbending training, and assured Monk Tashi that Aang was content and had time for games. The other monks disagreed with this, believing that Gyatso's affection for the boy was clouding his judgment. They decreed that Gyatso and Aang were to be separated and that Aang was to be sent to the Eastern Air Temple to complete his training. Aang overheard this and, having already been ostracized by his peers after his identity as the Avatar had been revealed, was shocked that the monks wanted to isolate him from his last and best friend. Not knowing that Gyatso intended to fight this decision, Aang fled the temple with Appa and was subsequently frozen into an iceberg for one hundred years. Gyatso never saw Aang again.
After Aang ran away, the Fire Nation launched an assault on the Southern Air Temple, intent on slaughtering the temple's population. Gyatso managed to fend off and defeat many firebenders, despite their enhanced abilities as a result of Sozin's Comet. However, he eventually fell, along with the rest of the Air Nomads.
A wooden statue of Monk Gyatso was placed in the courtyard of the Southern Air Temple before the entrance of the hallway that led to the Air Temple sanctuary, celebrating his airbending prowess. Gyatso's memory lived on in the form of Aang, who grieved his death greatly, going into the Avatar State upon discovering his corpse in the rubble of a destroyed building within the Southern Air Temple. When Aang was trying to open his chakras, he needed to release all his grief in order to open the air chakra located in his heart. When Aang laid out all his grief in front of him, all the Air Nomads with Gyatso at the front, appeared before him. Thanks to Pathik's guidance, Aang was able to cope with the pain that Gyatso's death and the demise of the Air Nomads had given him, and he was able to let go of it.
Gyatso had a great sense of humor; he often played pranks on his peers with his pupil, Aang, and even used his airbending to cheat during a Pai Sho game. Though he often played around, Gyatso was also a wise man, and one of the five members of the Southern Air Temple's Council of Elders. He was a kind and understanding individual who was well-liked by other students in the temple. Gyatso held a great adoration for Aang, desiring to keep him close by his side and shelter his childhood to the point that the council saw fit to separate the two. He also procrastinated in his training when he was younger, as seen when he did not practice his gliding, instead choosing to race with Avatar Roku.
Gyatso was Aang's father figure and mentor. The two shared a close bond and were often found having fun together. They used airbending to play pranks on the other inhabitants of the Air Temple, such as interrupting the Council's meditation by throwing fruit pies at them. Aang loved Gyatso like a father, and was devastated when the Elders told him he was the Avatar and would need to leave Gyatso. This, in fact, was one of the reasons Aang ran away from home. The full extent of Gyatso's reaction to this is not known, but he was shown to be deeply worried when he read Aang's goodbye letter.
When Aang returned to the Southern Air Temple one hundred years later, he accidentally stumbled upon Gyatso's skeleton, causing him to cross over into the Avatar State, overcome by rage, grief and guilt. Aang would continue to feel guilty about running away and "not being there for his people when they needed him most" until Guru Pathik helped him to deal with this pain and to let it go. A mirage of Monk Gyatso and the other airbenders played before his eyes when he thought of them and they disappeared into the clouds as Aang let them go.
Avatar Roku first met Gyatso when he journeyed to the Southern Air Temple in order to learn airbending. The two shared a small rivalry with each other, each of them trying to outdo the other by using air gliders to race each other. The two were great friends, even later in life. When Avatar Roku was showing Aang how the Hundred Year War started, he noted that "some friendships are so strong that they can even transcend lifetimes", citing his experiences with Gyatso as evidence.
Gyatso had a good relationship with the other Air Nomad boys, who were rather fond of him. He was regarded highly as an excellent airbender and was chosen amongst the other men to be on the Council of Elders. However, the other monks believed he was holding Aang back in his training, which resulted in Gyatso being forced to separate from Aang.