Paul Gardner had some big skates to fill following in the footsteps of his famous father Cal Gardner and older brother Dave, a star with the Toronto Marlboros who also played in the NHL. But he viewed it as a mere challenge, not an obstacle and he responded by eclipsing most of his older brother's scoring marks. After leading his local Leaside team in scoring every season, Gardner moved up to play Junior B with the St. Michael's Buzzers in 1973-74 where he tore up the league scoring 87 goals and 131 points in just 44 games. The following year he moved on to the OMJHL with the Oshawa Generals. His rookie season produced decent numbers, including 63 points. While he certainly did not have the same kind of offensive rookie year his brother Dave enjoyed four years earlier, he more than made up for it in 1975-76 when he potted 69 goals and 144 points in 65 games.
Gardner was selected in the first round, eleventh overall in the 1976 NHL Amateur Draft by the Kansas City Scouts. Later that summer the organization announced it was moving out of Kansas City and relocating to Denver where they became the Colorado Rockies. Gardner joined the team that fall and averaged almost a point per game, scoring 30 goals and 59 points in 60 games, finishing second in team scoring to Wilf Paiement. He again reached the 30-goal plateau in 1977-78 despite being limited to just 46 games. He reached 30 goals for a third straight year, notching 23 with Colorado before being traded to the Toronto Maple Leafs where he tallied another seven. Gardner had thought being sent to Toronto would help elevate his game, but he was never able to work his way into the lineup on a regular basis and found himself playing most of the time in the minors.
In November 1980, Gardner was dealt to the Pittsburgh Penguins where he spent the four most productive years of his NHL career. He was the victim of a vicious on-ice assault in a game against the Winnipeg Jets when he was sucker-punched from behind by Jets' goon Jimmy Mann. The hit broke his jaw and put him out of action for six weeks. Reflecting back on the incident years later Gardner felt the punishment did not fit the crime. "It just didn't seem fair that I missed 21 games because I was hurt, and he (Mann) got a 10-game suspension," Gardner said. He had been having a great offensive season and was on pace to hit 50 goals for the only time in his career. Eddie Johnston, the coach of the Penguins at the time said he was most disgusted by Jets coach Tom Watt, who he said most certainly told Mann to go after Gardner as a retaliation for Gardner's earlier crosscheck on Doug Smail.
Gardner returned from the injury and played in Pittsburgh until signing as a free agent with the Washington Capitals in the summer of 1984. He played 12 games with the Caps and made one final brief two-game stop with the Buffalo Sabres in 1985-86. He had two solid years in the AHL with Binghamton and Rochester, winning the league MVP in 1985 and 1986.