[3] Gillen was born in Arlington Heights, Illinois, and attended St. Viator High School,[3] where he lettered in football, basketball, and track.
[8] In 1984, Gillen headed to the United States Football League, where he played the rest of his professional career.
He joined the Chicago Blitz that year, where he played middle linebacker and was used once on a kickoff return for 11 yards.
[5][9] Gillen lives in Glen Ellyn, Illinois, with his wife, Wendy, and three children, John, Conor, and Grace.
[4] Both of his sons played college football: John for the Wisconsin Badgers[1] and Conor for the Fighting Illini.