He is best known for composing the musical You're a Good Man, Charlie Brown, based on the Charles M. Schulz comic strip Peanuts.

[1] The latter was also recorded in a smooth jazz version by David Benoit in May 2000, shortly after Charles M. Schulz's death, on an album entitled Here's To You, Charlie Brown: 50 Great Years!

[1] Gesner found work in New York City as a writer and composer for the Captain Kangaroo in 1955 and Mister Mayor in 1964 television programs.

[9] The stage adaptation of the concept album, entitled You're a Good Man, Charlie Brown,[10] went into rehearsal in New York City on February 10, 1967.

[29] Some of Gesner's well known compositions include "The Ivy League Look", a song for the Triangle Club,[30] and "Little Known Facts", a musical number from You're a Good Man, Charlie Brown.

[3] As an actor, Gesner performed regionally in theatre productions, including 1776, Lend Me a Tenor, and Carnival!, among others.

[31] He sang on the original concept album of You're a Good Man, Charlie Brown as Linus van Pelt and sang with Barbara Minkus on four bonus tracks on the original cast recording of the Off-Broadway cast of You're a Good Man, Charlie Brown.

[32] Gesner wrote for NBC Experiment in Television in 1967, shortly before completing You're a Good Man, Charlie Brown.

The 1968 television series The Ed Sullivan Show used Gesner's songs "You're a Good Man, Charlie Brown" and "Happiness", both of which had become hits at the time.