Gabriel is a talented young winger and the son of former Republic of Ireland international Joe O'Cearuill.
According to reports, Amorim met with Gabriel earlier this week to discuss his integration into the senior team's training sessions.
Before being added to the senior squad, Gabriel watched Manchester United's opening Premier League match against Arsenal from the directors' box at Old Trafford on Sunday.
Gabriel has also been informed that he will be training regularly with Amorim's squad throughout the season.
In a separate development, Gabriel made his full debut for Manchester United's U18 team in a win over Everton on Saturday. However, he will not be participating in the FA Youth Cup this season due to his age.