The 18-year-old has put pen to paper on a four-year contract at Elland Road, which will run until June 2026, and his arrival comes after Kalvin Phillips completed his switch to the Citizens for an initial £42m.
Speaking to LUTV on his arrival at the club, Gyabi said: "I am really excited! Obviously I have seen that they've given the young players a lot of opportunities in the first team here.
"I think it was nine debuts in the Premier League last season, so that's really good! I know a bit about the club, it's a great club.
"The fans are what has attracted me the most. I played here last season and the fans were crazy in an Under-23s game! So, I can't imagine how that's going to be in a full stadium."
Gyabi moved to Man City from Millwall's academy in 2018 and developed into a key player for the club's Under-18s team who won the 2020-21 Premier League North title.
The midfielder went on to record 10 goals and five assists in 39 appearances at youth level for the Citizens, including 10 appearances for the Under-23s side who won the Premier League 2 title last season.
Gyabi has also featured at Under-15, Under-16 and Under-18 level for the England national team, winning a total of 12 caps. body check tags ::