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.
The critic Paul Driver has written of Gruber: ‘Neo-romantic, neo-tonal, neo-expressionistic, neo-Viennese: he isn’t any of those things, so much as a sentient (and downright accomplished) composer who keeps responding to whatever musical stimulus, be it highbrow or lowbrow, 12-tone or 7-tone, bitter or sweet, that comes his way’.
!, a 'pan-demonium' for chansonnier and orchestra (or large ensemble) on poems from allerleirausch, a collection of children's verse by his friend, the absurdist and Viennese-dialect poet H. C. Artmann, which he performed as singer around the world in the following few years.
He and Schwertsik shared a 'Composers' Portrait' feature at the 1979 Berlin Festival, and Gruber has subsequently been ranked among Austria's leading composers.
As a performer (conductor, singer, bass player) he has been involved in music by Peter Maxwell Davies, Hanns Eisler, and Kurt Weill, and made notable CD recordings of the latter two composers.