According to the Guardian, that is because the final paperwork to complete the £27m deal hasn't yet been fully signed off - the transfer was initially agreed back in early August.
As such, Gabriel is not yet eligible to play for the club, although it is suggested that manager Pep Guardiola is hoping everything will be done in time for the youngster to be registered and take his place in the matchday squad against Everton before Friday's deadline for the game.
The ex-Palmeiras starlet, who has been taken under Neymar's wing in Brazil's national team, arrived at Manchester airport with his family a week ago. Having completed two sessions to ease him in, Gabriel then trained with the City first-team for the first time last Thursday.
Big things are expected of a player who has already scored 32 senior career goals for club and country, as well as three for Brazil's Under-23 team en-route to an Olympic gold medal in the summer. But the person who is expecting the most from Gabriel is himself.
In an Instagram post earlier this week, he spoke of a desire to conquer the world after emerging from humble beginnings in favelas of Sao Paulo, all the while trying to make his mother as proud as he can.
"I will be one of the favelados that will conquer the world. I will give my mother so much reason to be proud," he said.
For one so young, Gabriel certainly seems to have the right mentality and drive to make his dreams come true and succeed at the highest level. And veteran former Palmeiras team-mate Ze Roberto had many kind things to say about his temperament during the summer.
"In addition to the spectacular talent, I have not seen anyone more focused on football than him. That's the hardest thing to see in a player from the academy. He does not let it go to his head and he is not dazzled by fame," the ex-Real Madrid and Bayern Munich midfielder said.
"After Neymar, Gabriel Jesus is the best I've seen emerge from Brazil in these 10 years."
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