Definition - What does Gu mean?
Gu is a Sanskrit root word that, in the context of yoga, is often translated as “darkness.” Gu also means “to go,” “earth,” “water” and “cow.” Yogis know gu as the root of the word, guru. The root word, ru, is typically translated as “remover.” Guru, therefore, means "the one who removes spiritual darkness."
Verse 16 of the "Advayataraka Upanishad" refers to this definition of guru: “The syllable, gu, means darkness, the syllable, ru, he who dispels them. Because of the power to dispel darkness, the guru is thus named.”
Yogapedia explains Gu
Some scholars disagree with the definition of the syllable, gu, as darkness, calling it "folk etymology." Regardless, the “remover of darkness” definition highlights the significance of the guru's role as more than a simple teacher. The guru is a teacher of spiritual knowledge, a mentor and a guide along the path to enlightenment.
In Hinduism, Buddhism, yoga and related traditions, the guru also helps to perpetuate the teachings through a system known as the parampara or the guru-shishya tradition, which is the passing down of knowledge from one guru to the next. It's through this system that yogic teachings have survived throughout the centuries.