He is best known for his work with Badlands, in addition to his stint with Black Sabbath in the mid-1980s and recording most of the vocals on Phenomena's Dream Runner album.

In 1986, Black Sabbath started touring for the Seventh Star album when after only a few shows, singer Glenn Hughes got into a fist-fight and lost his voice due to the related sinus and throat injuries.

Also, in an interview Martin revealed that the sinister laugh heard on the track Nightmare is in fact Gillen's voice.

During the time of the Seventh Star tour, Gillen was asked by project director and co-producer Wilfried F. Rimensberger to join Mel Galley's Phenomena for the recording of the album Dream Runner, which features vocals from Glenn Hughes, John Wetton and Max Bacon.

In 1988, Gillen formed Badlands with Lee and recruited his friend Eric Singer with whom he had played in Black Sabbath.

He joined forces with drummer Randy Castillo and Iggy Pop band members Whitey Kirst and Craig Pike under the name Cockfight.

[5] He also joined the band Terriff, led by guitarist Joe Holmes, fresh off his stint with David Lee Roth on the A Little Ain't Enough tour.

In 1993, Wilfried F. Rimensberger was planning a remake of his first Metal Hammer Loreley Festival but this time specifically to stage Phenomena's first ever live performance, with Gillen on vocals.

He first showed symptoms of the disease around 1990, and according to his Badlands bandmate Jake E. Lee, "in between the first and the second record, he started getting really thin and didn't look quite as healthy".