The England U19 international completed his move last week.
Coventry chief executive Tim Fisher said: “Gael is a great talent and we would, obviously, have liked him to stay here at Coventry.
“But when the Premier League comes calling it becomes increasingly difficult to stand in the player’s way. We would have liked him to have developed further here at Coventry City but that wasn’t to be.
“So we have acted in the best interests of the club and have agreed a fee that we feel reflects his value.
“All of the money will be reinvested across the entire football side of the business – from youth development to the first team.
“Finally, we would like to wish Gael all the very best for the future.”