advice what transport should I take from citygate outlet to long ping yuen long in Hong Kong .

You can walk to the Tung Chung Station to take the Tung Chung Line. Get off at Nam Cheong station, where you can take the West Rail Line to Long Ping and Yuen Long Station.

You can still use the West Rail Line from Long Ping Station to the Nam Cheong, where you can transfer to Tung Chung Line. Get off at Hong Kong Station, which is about 600 meters from Lan Kwai Fong.