Gardner graduated in 2009 from Homestead High School. As a senior in 2008–09, Gardner was a team captain and defensive end on the football team which won the Wisconsin Division I state championship.
He received numerous top honors including 2008 Associated Press 1st-team All-State;[1] 2008 WFCA 1st-Team All-State;[2] 2008 all-North Shore Conference First;[2] and 2008 North Shore Conference Defensive Player of the Year.[2]
Gardner did not see field action in his first year (2009). As a sophomore or "redshirt freshman" in 2010, Gardner saw action in 11 games on special teams and as a second-string defensive end. He logged three tackles, including two tackles for loss and one sack.[4]
In 2011, he started 12 of 13 games, including the Fiesta Bowl vs. 13th nationally ranked Oklahoma state. Gardner recorded 35 tackles, 10 tackles for loss and 4.5 sacks. He was named to the All-Pac-12 Conference 2nd Team,[5] and received Stanford's Deswarte-Eller Award honoring the most outstanding (redshirt) sophomore. Gardner ranked 13th in the Pac-12 in tackles for loss per game, and 16th in sacks per game.[2] He had a season-high five tackles and one sack vs. California and a season-high 2.5 tackles for loss vs. Arizona.
Gardner started all 14 games of the 2012 season, including the Pac-12 Championship Game vs. UCLA and the 2013 Rose Bowl vs. Wisconsin. Gardner tied for second on the Stanford team with 14.5 tackles for loss and 7.5 sacks.[2] He also ranked among the nation's top 75 in both categories.[2] Against #2-ranked USC, Gardner won Pac-12 Defensive Player of the Week honors for registering 6 tackles, 3.5 tackles for loss and one sack, holding USC to 26 rushing yards.[6] Playing at #7-ranked Notre Dame, Gardner made two solo tackles, one fumble recovery, and one sack which caused a fumble that was recovered for a Stanford touchdown.[7] Gardner's season also included 49 tackles, 5 passes broken up, and 2 quarterback hurries.
He chose to return to Stanford for a fifth year in 2013, rather than enter the NFL Draft, but it ended early because of a torn pectoral muscle he suffered against Oregon State University.[8]