Gardner pitched in the Major Leagues from 1989 to 2001 for the Montreal Expos, Kansas City Royals, Florida Marlins, and the Giants. In his career, Gardner pitched in 345 games, posting a record of 99–93 and a 4.56 ERA. He appeared in 275 of his 345 career games as a starter. Gardner was also used as a long reliever and spot starter during the final years of his career. On July 26, 1991, Gardner no hit the Dodgers for nine innings but gave up a hit in the top of the 10th.
Gardner played the last six seasons of his career with the Giants. He shared the 2001 Willie Mac Award with Benito Santiago, which recognized each of their spirit and leadership.
Gardner attended Clovis High School in Clovis, California and California State University, Fresno, where he met his wife, Lori Gardner, an All-American softball pitcher. He and Lori have 2 sons. Lori Gardner struggled with liver cancer for several years during her husband's tenure with the Giants before dying from the disease in 2003. The Gardner family remains active in the Donate Life America organization.