Si(師) means "teacher" and fu(父) means father.
Sifu actually means someone who teaches you a kind of skills or techniques, which can be a kind of material arts, a kind of musical instruments, etc.
Sifu is not only your teacher, but also your father.
We have a idiom among us:
That means "If Sifu teaches you for one day, he is already your father for your whole life."
So we have to respect to our Sifu, just like we have to respect to our dad.
Similarly, SiGung (師公) is the Sifu of your Sifu,
Gung actually means "Grandfather" in Cantonese, and you got it.
This is my Sifu, Sifu Leung.