The 21-year old gloveman has signed a three-year contract with the Belgian Pro League outfit after passing a medical at the Jan Breydelstadion earlier this week.
"He has been playing three years for the Australian top team Central Coast Mariners," Gretarsson said. "Despite his young age he has gathered a lot of experience and he has been elected twice as best goalkeeper of Australia.
"This year he has celebrated the title with his team in the A-League. Another asset of Ryan is his attitude and mindset."