It was only the second time in the competition that Los Blancos have conceded in the opening minute, Roy Makaay's goal after 10 seconds for Bayern Munich in March 2007 being the only other occasion an opposing team has shocked the Spanish giants straight from the kick-off.
In scoring, Gallagher put his name in the history books as the first-ever English player to score in a derby between Atletico and Real in all competitions. It was also the earliest-ever goal by an English player in a Champions League game and the earliest that Atletico have ever scored in the competition too.
So, the scene was evidently set for Diego Simeone's side to finally break their UCL hoodoo against Real, and for large parts of the game they looked like doing just that.
Although Carlo Ancelotti's men had more possession (62% vs 38%), played more passes - 856 attempted was their highest total in a single match in the 2024/25 Champions League - and had a higher passing accuracy, they rarely looked dangerous.
Eight of Real's starting XI had a hugely impressive pass completion statistic of over 90%, but it could be argued that much of their dominance of the ball was largely irrelevant given that they did nothing with it.
When it mattered, just three of their 10 shots were on target compared to eight from 17 for Atleti, who managed to make it a busy night's work for their old boy, Thibaut Courtois.
The reality is that on the night Atleti's front men needed to bring much more than just the hint of a threat and a presence up front, with Julian Alvarez often ploughing a lone furrow in that regard. His 94.1% pass success was the best for his team, as was his five shots in the match. Only three Atleti players didn't have a shot in the game, and one of those was keeper Jan Oblak.
Of the six Real Madrid stars who couldn't fire off a goal-bound effort Kylian Mbappe's presence in the list might be considered a surprise, but the Frenchman actually had a stinker. He didn't manage a shot of any sort in the game and only touched the ball four times in the Atleti box.
His 53 total touches in the game was only five more than Courtois, and a total of 33 passes across the 120+ minutes of play was also amongst the worst on show.
Vinicius Junior's missed penalty, the first time he's ever missed from the spot for Real Madrid in all competitions (not including shoot-outs), summed up his own nightmare evening.
Of his 15 dribbles only three were successful, with just eight of his 23 duels won and 11 touches in the box coming to nothing.
Losing possession on 26 separate occasions was at least 10 more than any of his team-mates, whilst three fouls committed evidenced a growing frustration.
Subbing him late on was a brave decision from Carlo Ancelotti though it was arguably the correct one in the context of what was at stake.
The visitors did still manage to create some chances in the match (8) though none could be classed as any that would trouble Oblak, whilst 14 chances from the hosts would appear to support the belief that they were profligate on a night where they ended a four-game winless streak against their near neighbours (3D, 1L).
Marcos Llorente, whose missed spot-kick in the shoot-out ultimately handed Real the chance to progress, shouldn't be too hard on himself in the cold light of day.
The 30-year-old, whose great uncle, the legendary Gento, won six European Cups with Real and whose father, Francisco, and grandfather, Ramon Grosso, also played for Los Blancos, was the most industrious of those players wearing red and white.
Aside from his five clearances he also won five of his six tackles, and his energy helped to drive Atleti forward from first until last.
As the game began to get stretched and players began to tire, Real looked a little ordinary and were very much there for the taking. As a result, the performance from Atleti is likely to be seen as yet another missed opportunity by club hierarchy.
In the end it was Real that progressed of course, though not before losing their fourth match in this season's Champions League - the first time in 16 years that Real have lost four European matches in the same season. Only in 2000/01 did they suffer more defeats in a single European Cup/Champions League campaign (5).
After what was a nail biting and somewhat controversial end, Los Blancos progression meant it was the fifth time that Atletico had been eliminated from the Champions League knockout stages (incl. finals) by Real Madrid.
Only Bayern Munich have a worse elimination record in the competition's knockout rounds (7 v Real Madrid).
The story of the game will be that Atletico just couldn't capitalise when they had the chance, and the history books will again show that Real Madrid somehow always get the job done in the Champions League.
