Gregor was born in Rakosliget, now a part of Budapest, and studied voice at the Liszt Academy in the Hungarian capital. He made his professional debut in 1958 in the Hungarian Army chorus, then sang as soloist with the opera houses in Budapest, Gy‹r and Szeged, where he became director in 1990.

The Times writes that Gregor resigned from the Szeged post as he was unwilling to let political pressure interfere with his artistic programming.

During the late 1980s he began to perform more outside Hungary — initially in France, Belgium and Canada, then with Oregon's Portland Opera, where he appeared frequently until 2002.

He also sang regularly with the Houston Grand Opera, where he made his U.S. stage debut in 1987 _88 as Varlaam in Boris Godunov. During the 1994-95 season he made his Met debut as Dr. Bartolo, singing alongside Bryn Terfel in Le nozze di Figaro. That season (his only one with the company) he also sang the Old Priest in the Met premiere of Shostakovich's Lady Macbeth of Mtsensk and Dulcamara in L'elisir d'amore.

Gregor reportedly died after an unnamed illness. In September, he had unexpectedly announced his retirement from the stage in an interview with the Hungarian regional daily newspaper D_lmagyarorszšg, explaining that he was "very tired."