Guanhao is an island in  Paradise where the World Government organized a training camp for future Cipher Pol agents.
The island is covered by a forest. The only known building on the island is a high tower with trees on a roof.
See also the associated category: Guanhao Residents.
World Government uses Guanhao as a training camp for future Cipher Pol generations. Rob Lucci and other CP9 members had trained on the island until they became strong enough to become full-fledged agents.
CP9's Independent Report
Lucci and other CP9 members returned to Guanhao after recovering in
St. Poplar.  Very Good pursued them but he was easily defeated. 
The tower on Guanhao bears a resemblance to the East Tower on Jiang Xin Yu in
Wenzhou.  The island may be named after a Chinese fan who asked Oda about the tower mentioned above. The fan's name is Wu Guanhao (吳冠昊).