In Dragon Ball Online , it is shown that as a scholar Gohan became devoted to studying and researching the nature of ki and begins researching the martial arts of the past, organizing them in his book Groundbreaking Science in Age 804 . The book became extremely popular and introduced the concept of ki to the general population of Earth sparking a renewed interest in the martial arts and fighting. One of the articles, " Katchin Can Be Cut! ", had an immense influence on his brother and Trunks leading them study swordsmanship which allowed them to develop their natural potential via intense training and as adults Goten and Trunks founded the Kikoukenjutsu Sword School . The renewed interest in the martial arts also lead to the founding of the New Turtle School by Krillin and New Crane School by Tien Shinhan . As Goku and Vegeta had left Earth to settle their rivalry in Age 801 after Goku realized his death was imminent, it was left to the remaining Z Fighters Gohan, Goten, Pan, Trunks, Krillin, and Tien to defend the Earth after the remnants of the Frieza Force invaded Earth following the death of Gohan's father-in-law Mr. Satan whom the Guard troops that lead the army assumed had defeated Frieza. Like he had in the past, Gohan resorted to using his old alter-ego Great Saiyaman to defend the Earth and managed to convince Pan, Goten, Trunks, Krillin, and Tien to disguise themselves with Saiyaman-like suits as well to hide their identities while combating the Frieza Force, however their ki-based fighting style became a hot topic and global interest in ki escalated. As a result, Gohan in his role as a scholar and Z Fighter ended up sparking a martial arts revolution on Earth and secretly continued to defend Earth with his family and friends as he had done for most of his life.