Gagner opened the scoring with a goal less than 5 minutes into the game and the Oilers never looked back. They added consecutive tallies by Ryan Jones, Shawn Horcoff and Alexandre Giroux to take a 4-0 lead midway through the second frame.
Andrew Cogliano also scored and Devan Dubnyk turned aside 33-of-34 shots for Edmonton, which began a five-game road trip with the victory.
Ryan O'Reilly got the only goal for the struggling Avalanche.
Brian Elliott started for Colorado but was pulled after yielding four goals on 19 shots. Peter Budaj entered and stopped 11-of-12 chances the rest of the way.