following his graduation from penn state, gillen remained at the school as an assistant coach for the 1990 collegiate season. he was a volunteer assistant coach with james madison while he was working on his master's degree. he returned to as an assistant coach in august 1993 and held that position until he signed with the metrostars in 1996. he was also an assistant at maryland university and syracuse university. in 2002, gillen became the girls’ head coach at toms river high school north in new jersey. he later returned to coach the boys' team at his alma mater toms river high school east, and is also an assistant principal at the school.