Am I missing something? Seems like Gyeonggu's confession was genuine in high school, was it not? What's the truth…
Gyeong Gu initially confessed to Seung Hyeon, whom he liked, but after Seung Hyeon didn't reply, he felt ashamed, because of his sexuality and hid that he was confessing to him, by saying that he was practicing his confession to someone else. Seung Hyeon knew that Gyeong Gu is gay and, to make fun of him, he told him and "pushed" him to confess to Mi Ji. It wasn't a genuine confession, he confessed under peer pressure, Gyeong Gu and Mi Ji were just friends.