The 14-12 disappointment at a sodden Canberra Stadium torpedoed captain Sam Warburton's hopes of a 'perfect 10' wins and an unbeaten tour, while handing Robbie Deans' Wallabies extra belief ahead of Saturday's first Test in Brisbane.
"I guess that winning attitude is going to help (the Wallabies)," Brumbies captain Peter Kimlin said when asked what he would take back to the Australia camp.
"Probably some knowledge about the way they play obviously. But I'm sure they have come up with some ideas already how to beat the (Lions)."
The Lions management will put out a markedly different, and better, team to play Australia at Lang Park, but may now wish they had put more thought into the side for the seemingly inconsequential tour match in Canberra.
Facing a second-string team robbed of most of its Wallabies, Gatland rolled the dice, flying in three backline players into the country a day or two before the match, and backing their individual class to overcome any lack of cohesion.
The gamble backfired, with former Wales great Shane Williams, debutant winger Christian Wade and centre Brad Barritt all largely ineffective as the Lions forwards failed to provide them with clean ball from set piece or the breakdown.
Shipping Williams in from Japan for a one-match cameo, a player retired from international rugby since 2011, may have seemed an inspired choice to romantics and sentimentalists.
The decision now smacks of hubris born of a barnstorming run of five consecutive wins over a succession of weakened tour teams.
Former coach Clive Woodward, who led the Lions to a 3-0 series loss in New Zealand in 2005, slammed Australian rugby for showing "contempt" in fielding weak teams in the tour matches.
But it was Williams who spoke of "almost an arrogance" around the Lions camp as he put his 36-year-old body through one day's training.
The comment may have been intended as a compliment but it resonated loudly throughout the second half in Canberra, where Gatland stubbornly waited until deep into the second half before unleashing a raft of substitutions.
The rejuvenated forward pack and the boot of replacement fly-half Owen Farrell belatedly brought the Lions back into the game, but the horse had already bolted.
Gatland at least will have more certainty when planning for the Test series, with few of his side at Canberra Stadium putting their hand up for selection.
Barring number eight Toby Faletau who contested hard throughout and the reassurance of Farrell's kicking, Gatland is likely to draw a line through the rest of the team and feel assured in his selection of Warburton for Brisbane after Justin Tipuric's struggles in the openside flanker position.
"If these Lions were starting to feel overconfident, the past 72 hours will have brought them back to earth," Woodward said in a column in the Daily Mail after defending Gatland's selections.
New Zealander Gatland may eventually thank the Brumbies for providing the 'wake-up call', but Deans is likely to be more grateful to the Jake White-coached Canberra side.
The Brumbies' territorial game, defensive pressure and fierce attack at the breakdown ultimately provided the result the attacking flair and running rugby of the Queensland Reds failed to deliver, and offered an endorsement in principle of Deans' own conservative game plan.
However, the first Test will ultimately be won between the ears, said White, who believes the Wallabies now have the mental edge for a match that will probably decide the series.
"The first Test is going to be massive on the weekend," he said.
"There's a massive psychological warfare that's going to take place in those 80 minutes because it's do or die."