Gillespie took over the reins of the UC Irvine Baseball program in 2007, becoming the program's eighth head coach. During his tenure, the program has collected 23 All-Big West honors, including three Big West Pitcher of the Year titles, while five Anteaters have amassed a combined 26 All-America accolades. In three seasons, Gillespie has guided UCI to a 126-54 (.700) record and currently ranked 24^th in victories among active coaches at the Division I level, Gillespie owns an 889-525-2 Division I career mark and a .627 winning percentage.

Gillespie's coaching fame includes a 20-year career at USC, where he guided the Trojans to 15 postseason appearances, four College World Series appearances (1995, 1998, 2000 and 2001) and a national title in 1998 for which Gillespie was named National Coach of the Year by Collegiate Baseball and the American Baseball Coaches Association.

Gillespie posted a 763-471-2 (.618) record during his tenure at USC, producing 28 major league players, including five who were voted to the All-Star Game in 2003 (Mark Prior, Barry Zito, Aaron Boone, Bret Boone and Geoff Jenkins).

His reign at USC began in 1987 and in just his second season, guided the Trojans to the West Regional, falling just one game shy of the College World Series. USC made four consecutive regional appearances, including winning the conference title in 1991. From 1993, Gillespie led USC to an impressive 10 consecutive postseason appearances that included back-to-back College World Series appearances in 2000 and 2001, marking the first time USC accomplished the feat since 1973 and 1974.

His teams won five Pacific-10 titles, garnered Pac-10 Coach of the Year honors four times (1991, 1995, 1996, 2002) and West Region Coach of the Year accolades twice (1996 and 1998) by the American Baseball Coaches Association.

In 2002, Gillespie directed his talents to the USA Baseball National Team, leading the team to a 27-3-1 record and posting the highest winning percentage (.900) in history until the mark was surpassed by the 2003 team. The national team reeled off a 21-game unbeaten streak at the end of the season, including the Haarlem Honkbal Week championship in the Netherlands where Team USA captured the crown with an 8-5 win against Cuba. For his efforts, Gillespie was nominated by the United States Olympic Committee as the baseball Coach of the Year. It was his second stint with USA Baseball, having served as an assistant coach with the 1997 USA team.

He is only one of two men (Arizona's Jerry Kindall) to both play for and coach an NCAA-championship baseball team. He was the left-fielder for USC's 1961 national-championship squad and coached the 1998 team to the program's 12th title.

Prior to coaching at USC, Gillespie coached at the College of the Canyons, where he built the program from scratch. In his 16 seasons at Canyons, Gillespie compiled a 418-165 (.717) record and won 11 Mountain Valley Conference championships, including six consecutive from 1981-86. He captured three state titles and finished as the California runner-up twice. His teams finished with 20 or more wins in 13 years of his tenure, posting 30-plus wins six times. His final squad won 41 games in 1986, the most-ever by a California community college at that time. His term included three California Community College Co-Coach of the Year awards.

Gillespie served as Canyon's athletic director from 1977-86 and taught English, physical education and health education. He earned his bachelor's (1962) and master's (1963) degrees from USC.

Gillespie and his wife, Barbara, have four children, Kelly, Mitch, Matt and Tiffany and have three grandchildren, Cade, Cole and Samantha.