Gabriel, 15, has told United he wants to cancel his registration with immediate effect. BBC Sport reported on Thursday that Real Madrid have a firm interest in the forward, with Bayern Munich, Paris Saint-Germain and Barcelona also known to be keen. He is understood to have left the UK on Thursday and travelled first to France before arriving in Barcelona.
The move carries little immediate transfer significance, since United retaining Gabriel's registration would in any case prevent him from joining another English club before the end of the season.
But if he obtains an EU passport, through either his father — a former Republic of Ireland international — or his Greek-Cypriot mother, he could sign for a club in the European Union once he turns 16 on 6 October.
Gabriel has not trained at Manchester United since scoring a hat-trick on his Uefa Youth League debut on 10 September. Tournament rules mean he cannot play for any side other than United in this season's competition regardless of where his club future lies. Sources close to the player were said to be furious that United did not register him for their Champions League opener against Sabah FK, which Michael Carrick's side won 4-0.
Carrick was again asked about the situation at his press conference on Friday, ahead of Sunday's Premier League match against Fulham, and was careful not to add fuel to the story.
"He's a young boy, JJ, and I think it's important we don't play anything out here," Carrick said. "With the exposure and scrutiny and everything, you got to be careful and be sensible and think of him as much as anything.
"The situation will work itself out, whatever way it's going to work itself out. Making sure he's OK is the important thing."
The saga has become one of the most closely watched stories in English youth football this week, following reports earlier in the month that Gabriel had formally requested to have his registration with United terminated. His trip abroad, whatever its ultimate purpose, is unlikely to calm speculation over where the highly rated forward's long-term future lies.