when will this happen?
Well the former crush shows up in chapter 34, this story is a total of 50 chapters so the whole thing with that guy is maybe a dozen chapters. If you are wondering he doesn't actually cause any real problems except making Geon super jealous which as I said makes Geon want to fight for his man even though he doesn't have to cuz Gyuwon loves him, so in a weird way that guy showing up ends up strengthening the relationship, but he is still a total asshat. The guy weasels his way in staying at Gyuwon's place and tries to feed him, so Gyuwon packs up some things and gets the fuck out of there and goes to stay with Geon.
I can say from here on out the progress continues, Geon stops trying to push Gyuwon away as much, and than Gyuwon's first crush shows up. Actually despite that guy being an arrogant pos who deems Gyuwon worthy of being fucked by him cuz of the weight loss, it actually makes Geon step up and not let the asshat take his man. Not that the guy had a chance because Gyuwon is devoted to Geon, but nothing wrong with seeing an uke stand up to the asshole who is trying to break the relationship up.