Simon Makienok headed in his first goal since signing on loan from Italian side Palermo to put Charlton ahead before Abel Hernandez levelled a minute from normal time and Icelandic international Gudmundsson snatched a dramatic winner.
Lozen was sent to the stands by referee Tim Robinson for his celebrations as the Addicks extended their unbeaten start to the season to four games.
In-form Charlton striker Tony Watt, who had scored in each of the Addicks’ previous three games, was forced to miss the-game with a calf injury.
Watt’s place was taken by El-Hadji Ba, who went into midfield, with Gudmundsson joining Makienok up front.
Experienced defender Chris Solly was an ankle injury victim, which gave midfielder Zakarya Bergdich his first Championship start and Jordan Cousins dropped in at right back.
Bergdich carved out Charlton’s first real chance when he cut in from the left and his low shot was deflected for a corner.
Lozen’s side were not helped when Cristian Ceballos was forced off with an ankle injury midway through the first half, after attempting to carry on following a challenge with Sone Aluko.
Ceballos was replaced by 17-year-old academy player Karian Ahearne-Grant. The youngster had a wonderful opportunity moments before half time but lifted his shot high over the bar from eight yards out.
Even the match ball was finding things tough and had to be replaced on 25 minutes after bursting.
Both the number sevens had decent efforts with Hull’s David Meyler having a long range shot blocked and Charlton’s Gudmundsson dinking a cheeky chip over the bar.
The Addicks went ahead in the 52nd minute when Gudmundsson raced down the right flank and played in the perfect cross for Makienok to head home his first Charlton goal.
Gudmundsson had a terrific chance to grab a second when he sprinted clear, only for Allan McGregor to stand tall and beat away the drive.
Substitute Isaac Hayden hit a post with a low shot and then hit another drive, which Mick Pope fumbled and Hernandez tucked away from close range.
Hernandez thought he had got a Hull winner when he headed in another Hayden cross deep into the eight minutes of time added on but it was disallowed for offside.
But Gudmundsson got on the end of Makienok’s superb knockdown from a corner to snatch all three points.