Complex sentence: Gregorian chant is a variety of plainsong that was standardized by Pope Gregory I in the 6th century CE, and represents the first revival of musical notation after knowledge of the ancient Greek system was lost.
Split into simpler sentences: Gregorian chant is a variety of plainsong that is named after Pope Gregory I (6th century CE). Plainchant represents the first revival of musical notation after knowledge of the ancient Greek system was lost.