I think Gyaban told Sanji that he doesn’t have Conquerors haki, and that some people are born with it and some people aren’t, he was not born with it. That’s why Sanji was so surprised, because he was certain that he had it but that apparently was way off, until his next fight. Sanji is gonna fight Sommers, and it will be in that fight where he unlocks his Conqueror’s spirit. This is how I see the scene playing out.

Sanji and Sommers have a tussle, but they eventually reach a stalemate, where Sanji can’t damage Sommers and Sommers can’t pierce him with his Thorns. But Sanji will see Robin bloody and cut up from Sommers’s thorns, he will completely ignore his fight with Sommers and will go to see Robin. Sanji will cry over her and apologize for letting this happen, Sommers like the complete dick that he is will stand there laughing at him. Sommers will pull out a projector snail and will broadcast Sanji crying over Robin to Maryjois where everyone laughs at him, but Sanji won’t care. 

Sanji will look at Sommers, the snail goes out of commission, Sommers’s knees will start to shake. He will break out in a cold sweat, he can’t move. Sanji with all of his anger towards Sommers, and love for robin will reach a boiling point. Sanji was awakened Conquers haki, and it’s very strong. 

What did Gyaban tell Sanji?