Gabriel is understood to have asked to leave United after being left out of the squad for last week's Champions League opener against Sabah, despite scoring five goals in his three appearances for the club's Under-21 and Under-19 sides since.
According to The Sun, Manchester United had planned to include the teenager in their matchday squad for Wednesday's Carabao Cup third-round tie against Brighton, where he could have become the club's youngest ever player, before the situation escalated.
Manchester City, Chelsea and Barcelona are all said to be monitoring Gabriel's situation, with Barcelona previously making a concerted effort to attract the striker, who spent part of his summer in Catalonia. Gabriel, who was included among United's Premier League squad numbers this season wearing the number 77 shirt, has not trained with the first team in two weeks.
United are working to repair their relationship with Gabriel and his camp, though sources at the club maintain that any move to Manchester City specifically "cannot happen." Gabriel has formally requested to be de-registered from his contract, which runs until the end of the season, though United would demand significant compensation should he ultimately leave, and well-placed sources say interested agencies have already been quoted substantial sums to represent him ahead of any transfer.
The rivalry between United and City at academy level has intensified in recent years. City signed Harrison Parker in what United sources previously described as a "revenge move," while United brought in David Eze and Karim Cassim from City's academy over the summer, and Darren Fletcher's twin sons, Jack and Tyler, moved to Old Trafford from City in 2023.
Gabriel, whose father Joe O'Cearuill is a former Arsenal academy player capped twice by the Republic of Ireland, has been given multiple opportunities to train with United's first team and came on during a pre-season win over Atlético Madrid in Stockholm in August.
Former head coach Ruben Amorim and director of football Jason Wilcox provided reassurances to the family as part of the club's charm offensive last season, but O'Cearuill's role in negotiations has reportedly complicated the club's efforts to keep his son at Old Trafford.