Gaspare Charles "Gap" Mangione was born July 31, 1938, in Rochester, New York, into an Italian family where music was a nightly presence — his father regularly hosted visiting musicians for pasta dinners followed by jam sessions in the Mangione home. His younger brother is flugelhornist Chuck Mangione, who would go on to a Grammy-winning career of his own; the two started performing together in 1958 as the Mangione Brothers Sextet/Quintet, gigging around Rochester before their group drew national notice. Gap studied at Syracuse University, earning a dual degree in liberal arts and music in 1965, and during his college years worked as house pianist at the 1,200-seat Three Rivers Inn Theater Restaurant, where he played in the house big bands backing touring headliners including Sammy Davis Jr. and Nat King Cole.

The turning point came on August 8, 1960, when the Mangione Brothers Sextet cut The Jazz Brothers for Riverside Records (RLP 335) with the direct backing of Cannonball Adderley, who produced the session himself and released it as "A Cannonball Adderley Presentation," with liner notes by Riverside co-founder Orrin Keepnews. The sextet on that date was Chuck Mangione (trumpet), Sal Nistico (tenor sax), Larry Combs (alto sax), Gap Mangione (piano), Bill Saunders (bass), and Roy McCurdy (drums) — a hard-bop lineup that held together, with some personnel turnover, for two further Riverside LPs credited to The Jazz Brothers: Hey Baby! (1961, RS-9731, produced by Keepnews, with Steve Davis on bass) and Spring Fever (recorded November 28, 1961, RLP 9405, liner notes by Ira Gitler, with Frank Pullara on bass and Vinnie Ruggiero on drums). Those three albums, all still in print in reissue form, are the closest thing to a "debut era" this project can point to for Gap Mangione as a genuine, hard-bop-schooled jazz pianist working alongside Nistico, Keepnews, and Adderley himself.

Gap's solo career began with 1968's Diana in the Autumn Wind (GRC 9001), a record notable at the time mainly for giving drummer Steve Gadd and bassist Tony Levin their first studio credits, with Chuck Mangione writing, arranging, and conducting behind Gap's piano. The album went nowhere commercially on release, but decades later it became one of the most fought-over records in hip-hop crate-digging circles: the title track's horn-and-vibraphone break has been sampled by Jaylib (the Jay Dee/J Dilla and Madlib collaboration) on "The Official," by Madlib solo on "The Mystery (Dilla's Still Here)," and by Slum Village on "Fall in Love," among others documented on WhoSampled — popular enough that a specialty label reissued the LP's two most-sampled tracks as a dedicated "Diana / Free" 7-inch in 2022, explicitly marketed to "DJ culture." Gap followed it with the big-band session Sing Along Junk (1972, Mercury), arranged and conducted by Chuck with an all-star horn section (Clark Terry, Joe Farrell, Frank Wess, Jerome Richardson, Mike Mainieri), then four albums for A&M across the mid-to-late 1970s: She and I (1974), Gap Mangione! (1976, dedicated to Cannonball Adderley, "who gave Chuck and me our first record date"), and the guitarist Larry Carlton-produced Suite Lady (1978) and Dancin' Is Makin' Love (1979), which pushed into the jazz-funk and disco end of the era's crossover sound with session players including Jeff Porcaro and Robert "Pops" Popwell.

After a 1989 reunion record with Chuck and Gadd, The Boys From Rochester, Gap formed the Gap Mangione New Big Band in 1990 — originally booked for a 30-week dance-club residency, the 14-piece outfit grew into what remains the premier dance and concert big band in the Rochester area. Recording now under his own Josh Music imprint, he released Takin' It Back (1994), Planet Gap (1997), Stolen Moments (2003), Family Holidays (2004), and the live Live In Toronto! (2015, credited to the Gap Mangione Quintet). He held a 32-year residency (1987–2019) at The Lodge at Woodcliff outside Rochester, hosted a six-part syndicated PBS series called Gap's Generation, and in 2004 received the Artist of the Year Award from the Arts & Cultural Council for Greater Rochester; he was inducted into the Rochester Music Hall of Fame in 2015. He remains active in Rochester's jazz community today, including — per his own professional profile — as president of the local nonprofit MusicRochester, Inc. He was photographed in 2018 introducing a number at a joint Rochester Jewish Film Festival/Rochester International Jazz Festival event, still performing into his 80s.

Gap Mangione's musical arc runs from straight-ahead hard bop through soul-jazz, jazz-funk, and big-band writing, anchored throughout by his role as a bandleader-arranger rather than a soloist chasing individual spotlight. The Riverside years placed him inside the Cannonball Adderley/Orrin Keepnews axis of early-1960s hard bop, trading choruses with tenor saxophonist Sal Nistico alongside his brother's trumpet. By the late 1960s and through the Mercury and A&M years, his writing — often in tandem with Chuck's arranging and conducting — moved toward heavier orchestration, funk rhythm sections, and pop-adjacent material, the kind of soul-jazz/big-band hybrid that made 1968's Diana in the Autumn Wind a commercial nonentity at the time but raw material for hip-hop producers decades later. His 1978–79 A&M records, cut with guitarist Larry Carlton producing and an A-list Los Angeles session roster, leaned further into jazz-funk and disco without losing the big-band horn writing underneath. Since 1990, that same big-band instinct has anchored the Gap Mangione New Big Band, keeping him in essentially the same arranger-pianist-bandleader role he has held since the Mangione Brothers Sextet formed in 1958.