Gabriel turns 16 on Tuesday and holds an EU passport, meaning he can potentially join another club within the European Union provided FIFA approve the transfer.
Strict regulations govern the international movement of players under 18, meaning Gabriel's representatives would still need to satisfy a number of requirements before any switch could be completed.
However, the fact that the youngster's camp have requested permission to leave United suggests they believe those hurdles can be overcome.
Manchester United remain hopeful of finding a solution that convinces Gabriel to stay. Club officials were surprised when his representatives requested a departure last month and have since attempted to resolve the situation.
Gabriel has not trained at Carrington for more than two weeks. Instead, he has maintained his fitness by working individually with a private coach in Castelldefels, Barcelona.
The teenager and his family have regularly visited the city, where he has also trained with local specialist coaches. A private coach from Manchester has occasionally travelled to Barcelona to work with him.
His situation has attracted serious attention from both Barcelona and Real Madrid.
Madrid have been working to put themselves in a position to sign Gabriel, although they are aware of the sensitivity surrounding any move involving a player who has only just turned 16.
Barcelona's sporting director Deco recently played down the club's involvement when asked about the youngster.
“There's nothing to say about this boy. It's normal we always have players linked with us. There's a huge market for that, even when they are young. We are not interested in any player in his situation.”
Deco said Barcelona first needed to understand exactly what was happening before considering any approach.
“We are not involved in that. First of all, we need to understand how the situation is before we try to do something. To speak about young, teenage players, it is not our business.”
Despite those comments, Barcelona have followed Gabriel for several years and remain interested in the Manchester United academy prospect.
United therefore face a race against time to convince one of their most highly rated young players to remain at the club.
Gabriel's birthday changes the transfer landscape, but FIFA approval remains essential. With Real Madrid and Barcelona both monitoring the situation, United's priority will be to find a resolution before the youngster's future is decided elsewhere.