The Maple Leafs roughed up Martin Brodeur in his return while winning their eighth game of the season, tying Washington and Pittsburgh for the most victories in the Eastern Conference.
Brodeur, the all-time leading goaltender in wins and shutouts, had been out of action for three weeks with an injured shoulder. He gave up five goals on 23 shots.
Lupul's first goal gave Toronto a 3-2 lead with 6:31 gone in the second stanza.
Lupul put the Maple Leafs in front for good 9:45 into the second and he scored again less than 4 minutes later. He has eight goals for the season, two off the league lead held by teammate Phil Kessel.
Kessel was held scoreless in four shots, but assisted on Lupul's second and third goals.
Gustavsson made 19 saves to get the win, but for the sixth time in seven appearances this season allowed at least three goals.