He played junior hockey for the Fort William Canadiens and the Thunder Bay Vulcans. The Detroit Red Wings chose him 58th overall in the 1972 NHL Amateur Draft. He spent two seasons (1972-73 and 1973-74) in the Wings system and played 20 games for the NHL team. In 1974-75 he jumped to the Michigan Stags of the World Hockey Association (WHA). After the team moved to become the Baltimore Blades and then folded Gruen ended the season with the Winnipeg Jets. He played the next two seasons in the WHA for the Cleveland Crusaders, Minnesota Fighting Saints, and Calgary Cowboys.