Gabriel's profile has rocketed recently after being included in United's preseason tour squad over the summer.
That call came after being named Premier League Under-18 player of the year last term - in addition to winning United's Under-18 player of the year trophy - despite playing several years below his age bracket.
He featured in a preseason game against Atletico Madrid in Stockholm in August, scored a hat-trick on his UEFA Youth League debut last week and was expected to make his competitive senior United debut in Wednesday night's Carabao Cup clash with Brighton.
The report comes from Telegraph Sport, but details of the stance of Gabriel and his family are unknown at this stage, after he was removed from first-team training last week.
Gabriel has sent a formal notice he wants to cancel his registration with immediate effect, and United can challenge the request as he's contracted until the end of the current season.
He turns 16 on October 6th, after which he can sign a scholarship agreement with other clubs if the situation at United remains unresolved.