Walkom, an on-ice referee in recent seasons, held the same position between 2005 and 2009.
"We are fortunate to have someone with Stephen's on- and off-ice experience ready to step in to this position," said Colin Campbell, the NHL's senior executive vice president and director of hockey operations, in a statement.
"From 2005 to 2009, Stephen provided tremendous direction and guidance to our team of officials as the league implemented several rule changes that brought more flow and speed to our game. That management experience, combined with the fact that he has been back on the ice as a referee for the last four years, will be of tremendous benefit to the League and the game."
Walkom joined the NHL in 1990 and has refereed a total of 965 regular-season and 139 Stanley Cup playoff games.
The 49-year-old also worked the 2002 Olympics, the 2004 World Cup of Hockey, and the 2002, 2004, 2010 and 2011 Stanley Cup finals.
Gregson decided to retire after four years in the position and will serve as a consultant to the NHL on officiating matters, including the league's participation in the Sochi Olympics in 2014.
"Terry had the unique ability to expertly manage the 78-man NHL officiating team," said Campbell. "His tireless efforts behind the scenes to ensure that NHL officiating was the best it could be on a nightly basis were seldom recognized publicly, but we greatly appreciate his dedication to the game both on and off the ice for the past 33 years. We are also pleased that he has agreed to continue to work with the officiating department on various projects."