its gonna be long. sit down, this is a bible study.
ch 3 - first practice date
ch 77 - at home practice date. smth happened then they almost kinda kiss. Kinosaki then put his finger on Gero's lips, stopping him. "Anything further than this... you'll have to do with your future partner, okay?"
ch 88 - practice date. gero literally competes with beppu to impress kinosaki. then, at the end, gero asks "What do you think about Beppu?" Kinosaki paused before smiling, slightly blushing, "...That's nothing that should concern you, right?" He was implying that Gero shouldn't be jealous, because he's not one of the bride candidates.
ch 100 - after an event with one of the girls, Gero asks Kinosaki if he had somewhere he wanted to go, together. Kinosaki reprimands him, "Isn't that just a date? ...That won't do, Gero-kun. That's not the kind of relationship we have, remember?"
Also in ch 100 Gero legit exploding from Kinosaki's lewdness
ch 128 - Gero angry at his grandma for poisoning Kinosaki. Kinosaki straight up looking at him with a shojo manga like expression
ch 129 - Wedding chapter. You really just gotta read the whole thing.
it's kinda hard to explain why, but guessing from a writing analysis standpoint, kinosaki is more likely endgame. there's a writing "rule" called Chekhov's Gun. if you keep mentioning something, you should make it important to the story. the manga keeps edging readers with "will they wont they" with GeroSaki a little too much for it to just be nothing. it's like telling the readers, "there's nothing here~ you don't need to look here, look elsewhere~"
im guessing the manga with build up to a final arc/conflict about Gero getting past Kinosaki's constant rejection.
now you might think, what if Kinosaki is just straight up not interested? i don't think it's this. if Kinosaki had no feelings at all, he would've treated Gero like how he treats Beppu (who's always proposing to him), which is straight up cold.
so that lead me to guessing Kinosaki likes him (maybe even long before the manga started, there are hints of Kinosaki knowing Gero from childhood. ch 137). But keeps running away from romance because he's going to die soon (hint from Kinosaki telling his brother to live longer than him in ch 50, Kinosaki narration "After all... I don't have much time left..." in ch 53, chest scar in ch 100, Kinosaki narration "Gero-kun, I know you'll do just fine... even when I'm not around anymore." in ch 129) "Gero-kun, I really want you to be happy no matter what!" (ch 155)
usually, the hardest one to romance is the true love interest. like the final boss
tldr its just a guess from someone who's read a lot of Romance lol
Dont know if you mentioned it exactly (since I don’t recall which chapters these moments happened in) but the Christmas special where Kinosaki mentions how they want to spend their Christmas with the dearest. Which is hinted to be gero and almost have Gero questioned what he means by that. And also the kabdeon (not really a big moment per se but it just stuck out to me as a really imitated moment between them.)

People are saying gero and saki has high possibility to end up tgt, but all the signals so far dont lead the story that way (even after they both rejected beppu dating allegation). Can someone tell me why youre so sure they would be end game?