Gabriel, 15, formally asked United to cancel his registration earlier this month, having already refused to train ahead of the Carabao Cup third-round tie against Brighton, a game in which he would have become Manchester United's youngest-ever player had he featured.
His representatives have since lined up talks with Bayern Munich and Real Madrid, and Spanish outlet Sport reports that his shortlist has grown to four clubs, with Barcelona and Paris Saint-Germain also credited with interest. It's understood Gabriel wishes to pursue his career in Spain.
Wayne Rooney, whose son Kai has played alongside Gabriel for years, said on Stick to United that the writing had long been on the wall: "I've seen JJ grow up because he's been in Kai's team throughout the years, and I think that anyone close to it knew that was coming.
"In fairness to Manchester United, they've done everything, they've done absolutely everything to keep him, to keep him at the club. I think it's been known that there's been a desire to go and play probably in Spain, I think, I might be wrong, but this is what you've heard over the years."
Rooney drew a parallel with his own experience as a young prospect at Everton to explain why United had given Gabriel special treatment in an attempt to keep him.
"The club have treated him probably differently to the other players because he is such a special talent. I know, I recognise, I understand that because I was that player at Everton and I was getting that treatment at Everton which was you're getting a bit more, your parents get given stuff, you're getting that special treatment more than the other players are getting. Which, as a club, you need to do to try and keep your better players."
United's all-time leading goalscorer was unequivocal about the talent the club stands to lose.
"He is a top player. His ability, his balance, he has shown he can go up the ages and score goals. The next step is first-team football where it's different, and you never know until you're in there. But he's such a special talent who understands the game really well.
"At some point this season he would have made his debut for the first-team, I'd imagine, so that's where it's disappointing."