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 such as Amsterdam's Concertgebouw, the Berlin Philharmonic Hall, the Vienna Konzerthaus, London's Queen Elizabeth Hall and Wigmore Hall, the Salle Gaveau in Paris, the Washington DC National Gallery of Art and the New York Miller Theater. 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. Conductors with which she has performed include Vladimir Ashkenazy, Erich Bergel, Michael Boder, Andrey Boreyko, Jorg Faerber, Vladimir Fedoseyev, Edward Gardner, Neeme Jarvi, Emmanuel Krivine, Ingo Metzmacher, Esa-Pekka Salonen, Jose Serebrier, Vassily Sinaisky, Stanislaw Wislocki and Lothar Zagrosek. 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.

Gülsin Onay holds the title of State Artist in her native Turkey, where for many years she has been official soloist for the Presidential Symphony Orchestra in Ankara. She is “Artist in Residence” at Bilkent University in Ankara and holds honorary doctoral degrees from Bosphorus University in Istanbul, and from Hacettepe University in Ankara. In 2007 she was honoured with the award of a State Medal by the Polish nation for her interpretation of the music of Chopin. The same year, the “Sevda-Cenap And” Music Foundation awarded her its prestigious 2007 Honorary Award Gold Medal. Gülsin Onay was named “Pianist of the Year” in the 2011 Donizetti Classical Music Awards. In 2014 she was awarded the Honorary Medal of the 42nd Istanbul Music Festival, and later Honorary Medal from the Bodrum Music Festival in 2018. Starting in 2015, the Turkish city of Tekirdağ, one of whose streets has been named after Gülsin Onay, has organised a annual concert series “Gülsin Onay Piano Days” in her honour.

Outside of her professional performing career, Gülsin Onay has been a tireless promoter of classical music, through her charitable activities, event organisation, teaching and broadcasting. Since 2003 she has been a “Goodwill Ambassador” for UNICEF, and regularly gives charity concerts to support and publicise their activities. She has been the Artistic Advisor of the International Gümüşlük Classical Music Festival and Academy since its foundation in 2004. Gülsin Onay takes a particularly interest in nurturing the development of young musicians, and has taught over 400 pupils. Gülsin Onay is a regular contributor to radio and television arts programmes, and since 2017 she has hosted her own weekly programme “In the Studio with Gülsin Onay” on Turkish national radio.