He founded the magazine Institutional Investor in 1967. He was publisher of the magazine until 1990, and editor-in-chief for three more years, although he sold it in 1987 for $72 million. He then concentrated on conducting, hiring Avery Fisher Hall in New York for his debut in 1982. He established the Kaplan Foundation which is dedicated to scholarship and the promotion of the music of Gustav Mahler. After personal research, he has twice recorded Mahler's Second Symphony: with the London Symphony Orchestra in 1987, and with the Vienna Philharmonic in 2002. Anthony Holden wrote of the 2002 recording: "This is not just a historic recording, indispensable to serious collectors; it is awesome music making that will have Mahlerians in raptures."
Kaplan owns the autograph manuscript of Mahler's score of the second symphony and published a facsimile edition of the score in 1986. Tim Page wrote in the The New York Times: "Only now will musicians, scholars and the general public be able to own a facsimile manuscript of one of the composer's symphonies." He also owns the autograph manuscript of Mahler's song, "Ich bin der Welt abhanden gekommen." A facsimile of this manuscript was published by the Kaplan Foundation in 2015. Both manuscripts are on deposit at the Morgan Library & Museum on New York City. He is co-editor of the new critical edition of the Second Symphony as part of the Complete Critical Edition of Mahler's works, to appear soon. He is a member of faculty of the Juilliard School (Evening Division).
Mahler's Second Symphony is the only complete work he conducts, although he has also recorded the Adagietto from Mahler's Symphony No. 5. He has been invited to conduct more than fifty orchestras, including the Vienna Philharmonic, New York Philharmonic, Deutsche Oper Berlin, Symphonieorchester des Bayerischen Rundfunks, London Symphony, Royal Philharmonic, St. Petersburg Philharmonic, Russian National Orchestra and Moscow State Symphony, Los Angeles Philharmonic and the Philharmonic Orchestra of La Scala.
In February 1994, Kaplan conducted the Melbourne Symphony Orchestra in Mahler's Second Symphony. At the post-performance dinner in the Great Hall of the National Gallery of Victoria, the keynote speaker was the then Prime Minister of Australia, Paul Keating, a noted aficionado of Mahler.
Kaplan's conducting has attracted criticism and praise: in December 2008, The New York Times reported that New York Philharmonic musicians had complained that his recent performance with the orchestra was a "woefully sad farce", and that he should not be allowed to conduct them again. Steve Smith, writing in The New York Times of the same concert: “His efforts were evident throughout a performance of sharp definition and shattering power. From the acute punch of the opening notes, every detail of this huge, complex score came through with unusual clarity and impeccable balance. Every gesture had purpose and impact, and the performance as a whole had an inexorable sweep. …It seems likely that no one is better equipped to reveal the impact of precisely what Mahler put on the page.” Private Eye magazine claimed that Kaplan had only been allowed to conduct leading orchestras after paying them large amounts of money. In 1996, Kaplan conducted the Philharmonia Orchestra and the Chorus of Wiener Staatsoper in the opening night concert of the Salzburg Festival by invitation from Hans Landesmann. Michael Walsh, in his review of the concert wrote in Time Magazine: “…the 10-minute standing ovation – nearly unheard of from a Salzburg audience – that followed the stirring, soaring strains of the closing choral ode. The truth is, the evening was a triumph for Kaplan.”
Kaplan is the younger brother of the late Joseph Brooks, an Academy Award-winning composer who was found dead at his New York City apartment on May 22, 2011 in an apparent suicide while under criminal indictment on multiple sexual-assault and rape counts.
- Gilbert E. Kaplan: "How Mahler Performed His Second Symphony". The Musical Times, Vol. 127, No. 1718 (May 1986), pp. 266–267+269+271
- Gilbert Kaplan: The Mahler Album. Kaplan Foundation in association with Thames and Hudson, New York/London 1995, ISBN 0-500-97421-7; new, expanded edition: Kaplan Foundation, New York, 2011, ISBN 978-0-8109-9833-9
- Gilbert Kaplan: "In One Note of Mahler, a World of Meaning". New York Times, 17 March 2002 (online)
- Gilbert Kaplan: The correct movement order in Mahler's Sixth symphony. Kaplan Foundation, New York, 2004, ISBN 0-9749613-0-2
- Gustav Mahler: Symphony no. 2 in C minor : Resurrection : facsimile. Kaplan Foundation, New York, 1986, ISBN 0-571-10064-3
- Gustav Mahler: Adagietto. Facsimile, documentation, recording. Gilbert E. Kaplan, ed. Kaplan Foundation, New York, 1992, ISBN 0-571-51322-0
- Gustav Mahler: Symphony No. 2 in C minor "Resurrection". Benita Valente, soprano; Maureen Forrester, alto; Gilbert E Kaplan; London Symphony Chorus.; London Symphony Orchestra. 1988
- From Mahler With Love. Gustav Mahler: Adagietto, from Symphony no. 5; Gilbert E Kaplan; London Symphony Orchestra. 1992
- Mahler plays Mahler. The Welte-Mignon piano rolls. Gilbert Kaplan, Executive Producer. 1993
- The Kaplan Mahler edition. 1996. (contains Symphony No. 2, Adagietto from Symphony No. 5, the Mahler piano rolls, recorded recollections of musicians who performed with Mahler, plus a CD-ROM part containing 150 pictures from The Mahler Album)
- Gustav Mahler: Symphony No. 2. Latonia Moore, soprano; Nadja Michael, mezzo-soprano; Gilbert Kaplan; Wiener Singverein; Wiener Philharmoniker. Deutsche Grammophon 2003
- Holden, Anthony (November 16, 2003). "Classical CD of the Week". The Observer.
- Mahler, Gustav (1986). Symphony no. 2 in C minor : Resurrection : facsimile (Facs. ed.). New York: Kaplan Foundation. ISBN 0571100643.
- Page, Tim (May 18, 1986). "A Masterwork Made Public". The New York Times.
- Ben Finane (Spring 2015). "Lost to the World". Listen 7 (1): 29–32.
- "Keating promoted culture as something to celebrate", Sydney Morning Herald, 15 September 2009.
- Daniel J. Wakin, Mahler Fan With Baton Cues Unrest in the Ranks, The New York Times, December 17, 2008.
- Smith, Steve (December 9, 2008). "A Centennial Moment for a Mahler Epic". The New York Times.
- Music & Musicians, Private Eye no. 1227, January 2009, p. 12.
- Walsh, Michael (August 5, 1996). "Mad About Mahler". Time Magazine 148 (7): 64.
- Interview about 2nd Symphony
- A musician of the New York Philharmonic Orchestra on Kaplan's conducting
- "Desperately seeking Mahler", The Economist