Elf released three albums before they joined Ritchie Blackmore in his newly formed band Rainbow in mid-1975. Gruber played on Rainbow's first album, Ritchie Blackmore's Rainbow. Soon after the album was released, Blackmore fired everyone except Dio.
Gruber was also in the early recording sessions on Black Sabbath's Heaven and Hell album, co-writing "Die Young," until Geezer Butler
heard Dio, and returned to the band.
Gruber played live with Gary Moore on his supporting tour for his album Victims of the Future, and featured on Moore's 1984 live album We Want Moore! In 1980 he formed Bible Black with former Elf and Rainbow drummer Infinite Metal Werkz. Gruber had also been working on an Elf reunion, even though such a prospect seemed unlikely given the deaths of Driscoll and frontman Ronnie James Dio; both of whom had been members of the band from foundation until dissolution.
Early in 2010 Gruber formed "ED3N"- a metal band in the classic rock genre. The band featured vocalist Jeff Fenholt and guitarist David Shankle, of DSG and formerly Manowar.
On April 18, 2013 it was announced that Gruber had joined the band Raven Lord. He was to replace Jamie Mallender for the bassist's position. However, Gruber departed the band shortly thereafter and was replaced by Lucio Manca. On November 7, 2013, Gruber announced via Facebook that he became part of a newly formed band founded by Csaba Zvekan called Zvekan.