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).

Gu was sponsored by Run Run Shaw to attend the Berklee College of Music in Boston in the early 60s. Upon graduation he returned to Hong Kong and worked for both the Shaws Brothers and Golden Harvest movie studios. Later he joined TVB as their director of music, a position he held until he migrated to Canada in the 1990s. Gu has composed over 1,200 songs in his career, many of his songs are now considered as classic CantoPop.

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.