Courtois spent three seasons with Atletico on-loan from Chelsea.
Gabi, now with Al Sadd, told AS: "It is painful to see Courtois in Real Madrid's shirt.
"It's weird to see someone you've played together and won prizes with now at a rival, that hurts."
"He is crucial and cannot leave, not that he wants to. I have spoken with him and I know that there have indeed been offers from Manchester United and Juventus. Diego is the ideal leader for us, it does not surprise me that they are now working on a new contract for him."