On some level, I feel like Gaon meant to hurt Yohan when he said, "Soohyun is my world". I mean, I know she's important to him. She's his best friend. His person. But she's not the world. He knows it. She knows it. Yohan thought he knew that for a second.
Gaon is probably the most emotionally intelligent person on this show. He understands people the most, imo. And he knew that Yohan was hurting but in the midst of them trying to work out their issues... in the midst of a conversation, Yohan jumped right to, "cut Soohyun from your life" knowing how important she was to Gaon.
And Gaon just figured, if Yohan could do that to him, then he might as well say something that rips Yohan apart. Because we all know Yohan wants to matter to people. We know he's grown addicted to Gaon's affection. And Gaon can't punch Yohan. He's not even sure if he can just leave without Yohan lashing out.
But by saying that someone else is more important to him than Yohan?
Kim Gaon disarmed Yohan to the point that Yohan couldn't even speak for a while, much less lash out.