The Diocese of Derry says it has no knowledge or record of the arrangement.
It remains unclear when the boy left Belize, which mission he came from, or where he is today.
No offences involving the Belizean teenager have been reported.
Gallagher pleaded guilty in October to offences involving sexual communication with minors, indecent images of children, and attempting to encourage sexual activity with a child.
He remains in custody and is scheduled for sentencing on December 9.
The Bishop of Derry has requested his removal from the priesthood.