Gillespie started out as an intern at ad agency J. Walter Thompson, New York. He then moved on to D'Arcy Masius Benton & Bowles, BBDO, Deutsch, and Ammirati & Puris, first as an art Director, later as a creative Director. After eight years working on the agency side he moved into directing in 1995. Based on the strength of his spec reel and agency experience, he gained representation by production company Fahrenheit Films in late 1995. One year later, he signed with Coppos Films. He has been with MJZ since 2000 and continues to work as a commercial Director, commonly working with cinematographers Adam Kimmel and Rodrigo Prieto. Following nominations in 2001 and 2002 in the Directors Guild of America Award category for "Outstanding Directorial Achievement in Commercials", he won in 2006 for his Ameriquest and Altoids commercials. He also won a Golden Lion Award at 2005's Cannes Lions International Advertising Festival, and two of his commercials belong to the Museum of Modern Art in Midtown Manhattan's permanent collection.