I bet they went to high school together or met during something like the university orientation. Like when Gunjoon went drinking with friends after the break up, ML went out to kinda look for him or when they had a dinner together and ML was surprised that Gunjoon does not remember him. He perhaps met Gunjoon when he was drunk or kinda watched him from afar as a classmate, being interested in him or even liking him but waited till he was single...
I bet they went to high school together or met during something like the university orientation. Like when Gunjoon went drinking with friends after the break up, ML went out to kinda look for him or when they had a dinner together and ML was surprised that Gunjoon does not remember him. He perhaps met Gunjoon when he was drunk or kinda watched him from afar as a classmate, being interested in him or even liking him but waited till he was single...