Case T-578/16: Judgment of the General Court of 8 September 2017 — Gillet v Commission (Civil service — Contract staff — Former employed worker under Belgian law — Allowance applicable pursuant to Articles 6 and 7 of Decision C(2005) 1287 — Recalculation of the amount of the allowance by the administration in the context of an update of management procedures — Act adversely affecting an official or other member of staff — Act which is merely confirmatory — Obligation to state reasons — Recovery of sums overpaid)