in 1990, gillen signed with the penn-jersey spirit of the american professional soccer league. he returned to the spirit for the 1991 season, but also played one game, for a total of 34 minutes, with the albany capitals. he then signed with the san diego sockers for the 1991–1992 major soccer league season. in 1993, he played for the richmond kickers of usisl. that fall, he returned to penn state to become an assistant coach. on february 6, 1996, gillen returned to professional soccer when the metrostars of major league soccer selected gillen in the sixth round (59th overall) of the inaugural mls draft. he played fifteen regular season games before leaving the metrostars at the end of the season. in 1997, gillen signed with the third division new jersey stallions of the usisl. he played with the team through the 1999 season.