Gülsin Onay began playing the piano at the age of three, and gave her first public recital on Turkish Radio when she was six. With the aid of a special state scholarship she studied with Ahmed Adnan Saygun and Mithat Fenmen, and then in Paris, where her teachers includes Pierre Sancan, Monique Haas and Nadia Boulanger.. She graduated from the Paris Conservatoire at the age of 16, winning the prestigious “Premier Prix du Piano”, and prizes in top international competitions, including the Marguerite Long-Jacques Thibaud (in Paris) and the Ferruccio Busoni (in Bolzano), quickly followed. Gülsin Onay's subsequent, truly international career has spanned 80 countries across all continents, from Venezuela to Japan. She has given concerts in the major musical centres of the world. She has performed as a soloist with such leading orchestras as Dresden Staatskapelle, Philharmonia Orchestra, English Chamber Orchestra, Royal Philharmonic, Japan Philharmonic, Munich Radio Symphony, St Petersburg Philharmonic, Tokyo Symphony, Warsaw Philharmonic and Vienna Symphony Orchestras. Gülsin Onay's many festival appearances include Berlin, Warsaw, Granada, Mozartfest Würzburg, Newport, Schleswig-Holstein, Singapore and Istanbul.

Gülsin Onay is acknowledged worldwide as the definitive interpreter of the music of Ahmed Adnan Saygun, the leading Turkish composer of the 20th century, whose works feature prominently in her concerts and recordings, and whose 2nd Piano Concerto (which she premiered in Turkey and abroad) was dedicated to her. Other contemporary composers who have dedicated works to Gülsin Onay are Hubert Stuppner, Denis Dufour, Jean-Louis Petit, Muhiddin Dürrüoğlu-Demiriz, Marc-André Hamelin and Bujor Hoinic.

Gülsin Onay's over 20 recorded albums to date illustrate the breadth of her repertoire as well as her interpretive power, with solo works and concertos from Haydn and Mozart to the contemporary era. Her 2007 CD featuring live concert recordings of Tchaikovsky’s 1st and Rachmaninov’s 3rd Piano Concerto has been acclaimed by critics and virtuosi alike. In 2008 CPO released her recording of both Saygun concertos, to widespread critical acclaim. For the 250 anniversary of Beethoven's birth, Gülsin Onay has recorded 2 CDs of the composer's works. Many of her concerts have been broadcast on European radio and television, and in the USA on National Public Radio. During the 2020–2021 coronavirus pandemic, Gülsin Onay has become a regular performer on social media, where her livestreamed concerts have received several million views.