The Sifu comes from the Chinese word "師父" or "師傅"
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:
一日為師,終生為父
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.