First, as lachinatown mentions, the headstone of your great-grandfather simply says that he was from Chungshan (Mandarin: Zhongshan) 中山. Chungshan was formerly a county; now part of it is the city of Zhuhai 珠海市. Second, as tyuti1668 mentions above, the gravestone of your great-grandmother mentions a village called Sui Yoong Hahng (Mandarin: Shuiyongkeng) in Chungshan. Note that on the headstone the characters are 水雍坑 but the second character seems to be incorrectly used in place of the similar character 擁.
The information from your relatives is a bit of a puzzle that I have been trying to figure out with the help of Google and various Chinese websites. Some of the words might have been transcribed incorrectly. Here are my conclusions:
Kong Tung Sang = Cantonese Gwong Doong Sahng = Guangdong Province 廣東省 Lue Hoi See = Cantonese Jue Hoi See = Zhuhai City 珠海市 Sui Young Huang = Cantonese Sui Yoong Hahng = Shuiyongkeng Village 水擁坑村
Try as I might, I can't say for sure what the rest of those notes is trying to say. But Sui Yoong Hahng is located in modern-day Heung Jau Kui (Mandarin: Xiangzhouqu)--Heung Jau District 香洲區 in Jue Hoi/Zhuhai, as tyuti1668 points out.
What would be best would be to get the Chinese characters for what your relatives were trying to write in those notes.
This is much better--this eliminates the guessing game.
廣東省 = Gwong Doong Sahng = Guangdong Province 珠海市 = Jue Hoi See = Zhuhai City 香洲區 = Heung Jau Kui = Xiangzhou District 華前鄉 = Wah Chin Heung = Huaqian Township 水翁坑 = Sui Yoong Hahng (note second character should be 擁) = Shuiyongkeng Village
kwoo25, I made a voice message to one of my spies on the ground in Chungshan moments ago in regard to the Wah Chin Heung as provided by Greg. My spy told me, and I've seen it myself, Jue Hoi See is turned on its head, would be completely unrecognisable from your grandfather's day. He, my spy, is confident Wah Chin Heung may still be there,because Heung Jau Kui may survive the big push for progress.