Mao Collection, Middle Volume. Radical: Hand (shǒu). Entry: gan. Kangxi strokes: 17. Page 447, Entry 11.
Collected Rhymes (Jiyun): Pronounced yan (rising tone). It is the same as the character xian.
Book of Former Han (Qianhan): Yang Xiong, Changyang Fu: To signal to the city and capture the towns.
Commentary: Gan means to raise the hand with the intent to attack or seize. The pronunciation is similar to the word xian in the term for the canopy of a carriage.
Additionally, the Corrected Characters (Zhengzitong) states that gan is a corruption of the character chan. The ancient versions of the Changyang Fu were written as chan yi. See the detailed entry for the character chan for more information.
Another theory suggests that the pronunciations of xin and xuan are similar. The character xian is commonly written as gan, which is distinct from the character chan.