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 set up the Kaplan Foundation dedicated to 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. He has bought the autograph of Mahler's score of this symphony and commissioned a facsimile edition of the score. 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).
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: 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. Private Eye magazine claimed that Kaplan had only been allowed to conduct leading orchestras after paying them large amounts of money.
Kaplan is the brother of the late Joseph Brooks, 73, Academy Award-winning composer who was found dead at his New York City apartment on May 22, 2011 in an apparent suicide following a criminal indictment of 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
- "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.
- Music & Musicians, Private Eye no. 1227, January 2009, p. 12.
- Interview about 2nd Symphony
- A musician of the New York Philharmonic Orchestra on Kaplan's conducting
- "Desperately seeking Mahler", The Economist