We probably won't get to what Gaban told Sanji for a while, but I think Sanji's CoC is repressed along with his Germa genetics. In Wano, Zoro started using CoC because Enma drew it out of him, so what's left for him is to start drawing it out by himself so he can start using it properly. I think Sanji is in a similar situation. Sanji's Power of Love, even if it is it's own thing, leans heavily on Germa genetics and partially transforms Sanji into what he was supposed to look like. He had a choice to tap further into his genetics and become stronger but chose not to because he was scared of losing himself. I think Gaban told Sanji that he is repressing something in him and that unless he wants to truly see if he has CoC or not, he will have to stop holding back.