He was selected by Toronto Blue Jays in the 2001 amateur entry draft, and played in their minor league organization from 2001 through 2003[3]
On December 15, 2003, Gassner was sent to the Minnesota Twins to complete a trade made on July 16, 2003 where the Blue Jays sent Shannon Stewart and a player to be named later (Gassner) to the Twins for Bobby Kielty.[4]
Gassner spent the entire 2004 season with the Rochester Red Wings of the International League. On April 16, 2005 made his major league debut for the Twins against the Cleveland Indians getting the win by pitching 6 innings giving up only one earned run. Gassner was to have only one more start for the Twins, spending most of the season with the Rochester Red Wings.[5]
Gassner missed the majority of the 2006 season with a left elbow strain[6] and spent the entire 2007 season with the Rochester Red Wings where he compiled a 6-12 record and a 4.95 ERA.
On June 7 Gassner made his first start for the Sea Dogs against the Akron Aeros earning a no decision after pitching 52⁄3 innings, allowing 6 hits and 2 earned runs.[8] He became a free agent at the end of the season. Gassner signed with the York Revolution of the Atlantic League for the 2009 season. He retired on August 26, 2009.