 He studied at the Vienna Hochschule für Musik, his composition teachers being Alfred Uhl, Erwin Ratz and Hanns Jelinek, and later Gottfried von Einem, with whom he also studied privately.In 1961 Gruber joined the ensemble "die reihe" as a double bass player, and became principal bass of the Vienna Tonkünstler Orchestra in 1963.In 1968, with his composer friends Kurt Schwertsik and Otto M. Zykan and the violinist Ernst Kovacic, he co-founded the 'MOB-art & tone-ART' ensemble, partly to perform their own repertoire (which included a short piece by Gruber, "Bossa Nova", which rapidly became a hit tune) and partly that of Mauricio Kagel.