Gillet has been banned for three years and seven months after an investigation into two defeats for Italian side Bari in 2008 and 2009.
Gillet was a mentor to both Mignolet and Courtois.
"Both owe Gillet a lot, and they are stunned by what has happened to him," a source close to the Belgian FA to the Daily Mail.
"He is vastly experienced and was always on hand to help them, with advice or a word of encouragement. He was very much their mentor. They set great store by his guidance, and it is very sad that such a distinguished career should end in these circumstances."