There was a press release from Dragon Air couple of years back which mentioned that passengers flying with Cathay Pacific group wouldn't  need transit visa for Hongkong. I don't know if it still applies or was just another marketing gimmick for a short period of time.

If you are flying with any other airlines, you need transit visa even though you have US GC.

Related Links:
http://www.dragonair.com/da/en_IN/aboutus/pressroomdetails?refID=ea3d7fa697f96110VgnVCM32000011d21c39____#CountryDD

http://www.sajha.com/sajha/html/openthread.cfm?forum=2&ThreadID=58581

http://www.immd.gov.hk/ehtml/hkvisas_4.htm#part2