Another straightforward afternoon looked in store for pole-sitter Hamilton in Monza when he built up a sizeable advantage in the race's early stages, but his decision to pit while the yellow flags were out prior to the race's halfway point ultimately proved his undoing.
The pitlane had been closed as Kevin Magnussen had abandoned his car nearby, yet Hamilton and Antonio Giovinazzi did not clock the warning light and came in for fresh tyres, which resulted in 10-second penalties for the duo.
Yet Hamilton only discovered that punishment was forthcoming after Charles Leclerc's huge crash resulted in the race being stopped shortly afterwards, meaning the Formula One drivers' championship leader had to pit after the restart and then resumed at the back of the pack.
While he guided his Mercedes back up to seventh, it was AlphaTauri's Gasly who took the first checkered flag of his F1 career ahead of McLaren's Carlos Sainz and Racing Point's Lance Stroll.
"Firstly, huge congratulations to Pierre, it's a huge result for him," Hamilton told Sky Sports.
"Being dropped by the top team [Red Bull] to beating the top team, I'm genuinely really happy for him.
"It wasn't meant to be for me today, but what doesn't kill you makes you stronger. I enjoyed the battle, onto the next one."
Hamilton's mood might have been worse had his championship rivals managed to take advantage of his error, but neither team-mate Valtteri Bottas nor Red Bull's Max Verstappen enjoyed great outings either.
Bottas slipped from second to sixth after a shocking start to the race, though he finished fifth, while Verstappen - who was down in 11th when the race was halted after Leclerc's crash - was later forced to retire.
Hamilton started the day with a 47-point lead over Verstappen - an advantage which remains the same, albeit with Bottas now second in the drivers' standings.
"I take responsibility for not seeing the boards," Hamilton admitted.
"But finishing seventh and a fastest lap, I'll definitely take it.
"Max Verstappen didn't score any points, so not a huge loss today."