Gratton played his minor hockey in his hometown of Brantford, playing for such programs as the Brantford Nodrofsky Steelers (same program Wayne Gretzky played in many years earlier) and the Brantford CKPC Knights. As a 15-year-old, he played one season with the Jr B Brantford Classics.
In 1991, Gratton was selected third overall in the OHL Priority Selection by the Kingston Frontenacs. After his rookie year in the OHL, Gratton received the Emms Family Award as rookie of the year. In 1993, he was selected third overall in the NHL Draft by the Tampa Bay Lightning.
Gratton made his NHL debut for the Tampa Bay Lightning on October 6, 1993 against the New Jersey Devils. He recorded his first career point (a powerplay assist on a Bob Beers goal) in his second game against the New York Rangers the next night. He scored his first career goal against Robb Stauber of the Los Angeles Kings in a 4-3 Lightning loss on October 20, 1993
In 1997 Gratton signed with the Philadelphia Flyers as a free agent after four seasons with the Lightning, earning a $9 million signing bonus. In 1997–98, Gratton matched his career high of 62 points and recorded a career-best plus/minus rating of +11. Gratton was traded back to Tampa Bay the next season.
Gratton served as Tampa Bay's captain during the 1999–2000 NHL season until his trade in March, 2000, when Tampa Bay traded Gratton to the Buffalo Sabres.
Gratton would play for the Buffalo Sabres from March 2000, till March, 2003, when the Sabres traded him to the Phoenix Coyotes. One year later on March 9, 2004, Phoenix traded Gratton, along with Ossi Vaananen and a second round draft pick (Paul Stastny) in the 2005 NHL Entry Draft to the Colorado Avalanche for Derek Morris and Keith Ballard.
After the 2004–05 NHL lockout, Gratton signed a one-year contract with the Florida Panthers. Gratton signed a two-year contract extension with the Panthers in March, 2006. He was traded back to the Lightning again on June 13, 2007, in exchange for a second round draft pick (Florida selected Jacob Markström with the draft pick).
During the 2007–08 season, Gratton scored 21 points in 60 games before tearing the acetabular labrum in his left hip which required season-ending surgery. Gratton re-signed with the Lightning for the 2008–09 season but was waived in December and was assigned to Tampa's American Hockey League affiliate, the Norfolk Admirals.
Gratton was claimed off of re-entry waivers on February 21, 2009, by the Columbus Blue Jackets. He played in six games for Columbus, recording an assist, before retiring in 2009.
Gratton is co-owner of the Florida Jr. Blades Organization (in the 29-team Empire Junior Hockey League).