Gu Gaa-fai (顧嘉煇; Yale: Gu Gà Faì), or Joseph Koo (born 1933) is one of the most repected composers in Hong Kong.
He had a pen name 莫然 for Mandarin songs. He is the younger brother of famous Chinese singer Gu Mei (顧媚).
In 1962, he composed his first hit (Mandarin Chinese) song 不了情 and later another hit song 郊道. In 1974, he wrote the first Cantonese TV theme song (啼笑姻緣) which was the first actually popular Cantopop song.