I found the official site of the Jiuzhou port. It's at
www.jzportpt.com. Chinese only, I think. If you're going to Shekou, then definitely take the ferry from Jiuzhou. Departure every half an hour. 80RMB, which is about half the cost of going to Hong Kong.
I also found a site with bus routes:
www.okzhuhai.com. #4 bus goes from Gongbei to the port. Should be very frequent - the site says every 8 minutes.