I don't know much about Gould's negative feelings about Horowitz, but I
recall a famous rumor/legend that Gould stumbled across an editing session
of Horowitz in the studio one night. The editors/engineers were having
trouble with a short badly recorded passage, so, with great glee, Gould
(allegedly, purportedly) ran to the piano and mimicked the passage a la
Horowitz so the editors could splice it into the Horowitz recording. Later
he loved to point to the Horowitz recording and whisper, "I'm on that, can
you find my playing on that?"