After becoming the youngest player ever to hit 50 Bundesliga appearances, bundesliga.com looks at the landmark moments in a precocious career which promises so much for the USMNT international.
18 January, 2020: Bundesliga debut
It was 18 minutes from the end of a 5-3 win against Augsburg when Reyna emerged from the BVB bench to make his bow in the Bundesliga. It might have made a bigger splash had Erling Haaland also not done the same and smashed a second-half hat-trick to help Dortmund come back from two goals down twice to take the points.
It was still momentous, though, for Reyna and US soccer. Aged just 17 years and 66 days at the time, the ex-New York City FC academy prospect became the youngest American to play in the Bundesliga, beating the record previously held by former Dortmund man Christian Pulisic.
Though Haaland stole most of the limelight, then-Dortmund coach Lucien Favre had been impressed with Reyna's contribution.
"In training you can see that he has something special," the Swiss coach said at the time. "If you can’t see that, you're blind. His movement is correct, he plays in the right way and he can already do a lot at his age."
18 February, 2020: UEFA Champions League debut
There was another landmark substitute appearance for the UK-born Reyna a month later when he stepped onto the pitch 23 minutes from the end of his club's Last 16 first-leg tie with Paris Saint-Germain, becoming the third-youngest player ever to appear in the competition's knockout phase.
He then teed up Haaland for the game-winner making him the youngest American to play and record an assist - his first for Dortmund's senior team - in a UEFA Champions League match, again at the expense of Pulisic.
"There are still a lot of emotions going through my head right now," Reyna told bundesliga.com after the game. "Obviously it was amazing to make my Champions League debut against Paris. It was a dream come true for me. With all those players on the field, and to get an assist was even better but the most important thing obviously was that we got the win. I'm so happy about that."
Reyna has gone on to make a further nine UEFA Champions League appearances, and tallied a first start in European club football's elite competition when he played the full 90 minutes in the 1-1 draw at home to Lazio on Matchday 5 of the 2020/21 group stage.
19 September, 2020: 1st Bundesliga goal
Reyna's maiden first-team goal had come in the DFB Cup against Werder Bremen in February 2020 - becoming the club's youngest scorer in the competition - but he had to wait until Matchday 1 of the following season before opening his account in the Bundesliga.
Again, Haaland scored a brace, but Reyna got the all-important opening goal in a 3-0 Matchday 1 defeat of Borussia Mönchengladbach that got his club off to a flying start.
"Overall, it was a really good night to score and get the win. Now we just have to kick on and perform just a little better going forward, but it was great," said Reyna.
"I only have to say a couple of words: he's the American Dream," added Haaland. "I don't need to say more." Soon, Reyna was certainly living the American dream.
12 November, 2020: USMNT debut
There cannot be many better ways to celebrate your 18th birthday than playing for your country at the highest level. Just a day before his landmark anniversary, Reyna did just that in a goalless draw with Wales in Swansea becoming the 12th player to make their USMNT debut under the age of 18. He also became the third member of his family, after dad Claudio and mum Danielle, to wear the US shirt.
Born in England during his dad's playing career, and with Argentine and Portuguese roots thanks to his father, Gio could have played for any of those three nations, but they were never an option.
"I'm an American kid. I grew up there. It’s my culture. It’s my home, it’s my family. I’m American through and through," he said.
"Gio and Claudio both kind of just move around the field in a very fluid way, and I think that's something that's a really big comparison of both of them. They're both competitive," said USMNT coach Gregg Berhalter, who played alongside Reyna Sr.
“I know Claudio was was really competitive, you can see Gio also has that competitive fire. In these games that we've been watching from Dortmund, you can see how badly he wants to win the games. So those are two probably major similarities that I see between them.”
16 November, 2020: 1st USMNT goal
Vienna will always hold a special play in Reyna's heart. No, not for the delicious Sachertorte or the world-renowned opera, but because it's the place where he scored his very first goal for the USA just four days after making his debut.
With just 18 minutes on the clock in Austria in a friendly against Panama, Reyna fired home a free-kick to open his side's scoring in a thumping 6-2 win and become - after Pulisic and Juan Agudelo - the third-youngest goalscorer in USMNT history at 18 years and three days old and showing he can take and make goals.
"When you have a player like that, there are a lot of different ways you can mould him and form him. You see that he's so adaptable, and that is what makes him so valuable to the team," Berhalter told bundesliga.com.
By the end of a bountiful 2020, it was no surprise to see Reyna named US Soccer's Young Male Player of the Year.
13 May, 2021: First title with BVB
It is a competition which already featured Reyna's name in the history books, long before he got his hands on his first piece of silverware with Dortmund.
At the age of 17 years and 82 days, Reyna became the youngest goalscorer in the DFB Cup for Dortmund when he struck just 11 minutes after coming on in the last 16 against Werder Bremen. "Gio's already really, really mature," said former Dortmund youth coach Otto Addo at the time. "He's already mentally fit and can draw on his full potential even when under pressure."
The pressure does not come much greater than a DFB Cup final, but having steered in two goals in a semi-final win over Holstein Kiel, it was only right that Reyna would be on the field when BVB beat RB Leipzig to lift the title in Berlin, securing the USMNT forward his first piece of silverware, and probably the first of many.
6 June, 2021: First trophy with USMNT
The 2021 CONCACAF Nations League afforded Reyna the opportunity to pick up his first major piece of silverware with the USA. With the coronavirus pandemic having moved the four-team tournament from its initial slot in April 2020, Reyna had had additional time to establish himself in Berhalter's squad, and such was his stature come the competition, the Dortmund youngster was in the starting XI to face Mexico in Denver.
One-nil down after just 62 seconds, Reyna got his third senior international goal - and his first of the tournament - when he pounced on the loose ball after a Pulisic corner had been headed against a post by former Schalke man Weston McKennie.
A McKennie goal later forced extra-time, and to cap a ding-dong encounter in Colorado, Pulisic slotted home a 114th-minute penalty to make the USA the first-ever CONCACAF Nations League champions.
"He brings a skill set where he's very comfortable on the ball," USMNT and RB Leipzig midfielder Tyler Adams said. "You can give him the ball in tight areas on the field. He manoeuvres with it. He can beat players one-on-one. And as of late, he's really showing off his his end product with a lot of assists and adding some goals to his game as well.”
27 August, 2021: 50th Bundesliga appearance
Reyna set not only a Dortmund record in a 3-2 win over Hoffenheim on Matchday 3 of the 2020/21 season, he also dislodged former Bayer Leverkusen midfielder Kai Havertz from his place as the youngest player to reach 50 Bundesliga appearances.
Reyna brought up the half-century landmark at the age of 18 years and 287 days in a game which, new record aside, also put the hypothetical seal on his status as a key player for Dortmund.
Watch: Half a century of Gio Reyna - and more to come
The USMNT man started all of Dortmund's first three games of the season under Marco Rose and at that rate, he will bring up a century in record time too, with his new club coach also developing new aspects of his game which make him increasingly versatile.
"Defending was always an area I needed to improve and I now have a coach who is pushing me a lot in this area, and he's expecting improvements from me," Reyna told Kicker.
Being more versatile will mean Reyna providing more options, though he is already looking like a player Rose and Dortmund simply cannot do without.