Gu Long seems to love giving the surname Hua (花) to his characters. I don't know if this is he most commonly used surname in GL's novel, but, he sure used it a lot. He also used it in his characters' name quite often.
Here are some I have compiled (with the help of some friends and reference from some posts in baidu messageboard).
Characters with 'Hua' surname
Hua Man Lou (花满楼) - Lu Xiao Feng Chuan Qi (陆小凤传奇)
Hua Man Tian (花满天) - Bian Cheng Lang Zi (边城浪子)
Hua Man Tian (花满天) - meantioned in Bai Yu Lao Hu (白玉老虎)
Hua Wu Que (花无缺) - Jue Dai Shuang Jiao (绝代双骄)
Hua Ye Lai (花夜来) - Bi Yu Dao (碧玉刀)
Hua Yue Nu (花月奴) - Jue Dai Shuang Jiao (绝代双骄)
Hua Cuo (花错) - Feng Ling Zhong De Dao Sheng (风铃中的刀声)
Hua Ping (花平) - Xiao 11 Lang (萧十一郎)
Hua Ru Yu (花如玉) - Xiao 11 Lang (萧十一郎)
Hua Bai Feng (花白凤) - Bian Cheng Lang Zi (边城浪子)
Aunty Hua/Hua Gu Ma (花姑妈) - Chu Liu Xiang Chuan Qi : Xin Yue Chuan Qi (楚留香传奇之新月传奇)
Hua Ling Ling (花灵铃) - Da Qi Ying Xiong Zhuan (大旗英雄传)
Hua Shuang Shuang (花双霜) - Da Qi Ying Xiong Zhua (大旗英雄传)
Hua Xun (花讯) - Ming Jian Feng Liu (名剑风流)
Hua Wu Yu (花舞语) - Nu Jian Kuang Hua (怒剑狂花)
Characters with 'Hua' in their name
Hue Tie Hua (胡铁花) - Chu Liu Xiang Chuan Qi (楚留香传奇)
Wu Hua (无花) - Chu Liu Xiang Chuan Qi : Tie Xue Chuan Qi (楚留香传奇之铁血传奇)
Wang Lian Hua (王怜花) - Wu Lin Wai Shi (武林外史)
Hong Lian Hua (红莲花) - Ming Jian Feng Liu (名剑风流)
Ji Zang Hua (姬葬花) - Ming Jian Feng Liu (名剑风流)
Jin Ban Hua (金伴花) - Chu Liu Xiang Chuan Qi : Xue Hai Piao Xiang (楚留香传奇之血海飘香)
Chang Hua (藏花) - Nu Jian Kuang Hua (怒剑狂花)