Gyeong-su was quarantined in the broadcast because Na-yeon, your sister accused him of being infected, because he had a scratch on his hand, which he got when he pushed the zombie out of the room you were in, Na-yeon was always implied with Gyeong-su because she thought she was superior to him, because he lives in a housing complex, and you as her sister, you never approved this. 30 minutes passed and Dae-su and Ms. Park intended to make Gyeong-su leave the broadcast room since he didn't seem infected or turned into a zombie, but he didn't want to leave.

You then entered the broadcast room to try to get him to leave the room there. While the rest of your friends were outside watching through the glass window, the room you were in was soundproof, so you couldn't hear anything outside.