I've recommended a Wattpad translation of the 'My Gear and Your Gown' novel but I see that recently the translator has changed her name and disguised the book's title. This will make it difficult to find the book using a search on Wattpad.
To help you find the translation here is a link to the translator's profile on Wattpad:
https://my.w.tt/qLvBBLEdLbb
UPDATE (Jan 8 2021): The Wattpad account owner has taken down the translation but plans to launch a separate site to host all of their translations behind a registration wall. There will be no cost for registering. The one book that remains on their Wattpad is actually a Q&A on how to register, etc.
The novel has a similar timeline to the series. I've written a synopsis below.
** WARNING: SPOILERS!! **
(Even though the series differs from the book in important ways, there are still many things that are the same.)
A general comment: If they'd cast the part of Itt in the series to match the look of the character in the book, the part would have gone to Joss Way-ar (https://mydramalist.com/people/21407-joss-way-ar-sangngern). Pai, on the other hand, is described as skinny with a "Korean idol look". He is also considered very handsome but because he grew up as a nerd wearing thick glasses he lacks self-confidence.
1. At the beginning of Book 1, Pai encounters Itt at the start of his first year at university. Their relationship is tense at first but it gets much better, to the point that Itt starts to flirt with Pai. Pai was wounded by his one-sided love for Itt in high school so he resists Itt's attempts to get closer in college. The flirting gets noticed by other students though, and eventually social media at the school is blazing with rumors of #IttPai. Pai's interactions with Mr. 950 are much more extensive than in the series.
2. Book 1 stays within the college time frame and ends on the night that Pai wins the Moon and Star competition. Mr. 950 has promised that he'll reveal his identity after the competition and instructs Pai to meet him at the Photo Club room. Pai shows up and discovers that Mr. 950 is Itt, and Itt confesses his love with a little bit of stagecraft and some incredibly sweet dialog. At one point Itt actually gets on his knees before Pai and asks Pai for 30 days to make Pai fall in love with him and become his boyfriend. Pai agrees to this 'flirting' period. This ends Book 1.
3. Book 2 starts with the flashback to their high school years. It covers the animosity that Itt initially feels toward Pai as well as Pai's one-sided love for Itt. There's still the fight over the Club Room, and Pai ends up tutoring Itt at the request of a teacher, but there is no deal struck with Pai over ownership of the Club Room -- the tutoring and Club Room are entirely separate things in the book.
4. The tutoring sessions start out rough, mainly because Itt still despises Pai and does nothing to hide it. Then one day Pai helps him with an assignment and Itt realizes that Pai's a pretty nice guy. Not only that, but he knows how to explain things better than any teacher ever did, and he never judges Itt. Itt starts to take the tutoring seriously and Pai becomes the only person he has ever told about his dream to become an engineer or his own lack of self-confidence. Pai's acceptance and encouragement motivate Itt like never before, and Itt eventually starts to like Pai (but does not tell him.)
5. The night they're locked in the Club Room and have to sleep together is all the result of a ruse by Itt. It's the last time they'll have a tutoring session before the exam, and this means they're unlikely to ever speak again. (It turns out they've never told any of their friends about the tutoring arrangement or that they've gotten on friendlier terms, so outside of the tutoring they never acknowledge one another.) The room has no windows so after lights-out it is pitch black. Itt and Pai lay side by side. The AC makes the room cold and when Pai starts shivering Itt startles him by reaching over and pulling him into his chest to keep him warm. They fall asleep like that. Itt reflects on how safe Pai makes him feel, during a time in his life where he's been unable to get emotional support from anyone else, including the girlfriend who has just broken up with him or his best friend.
6. In the book it's Itt who actually wins the competition for the rights to the Club Room, but as he lays in the dark with Pai he says that they can continue to share the room. He does so on one condition: that Pai will someday grant for Itt any request that Itt makes. Itt can't decide what he wants to ask for, so he asks for the Lab Coat earring as collateral to ensure that when the time comes, Pai won't back out of the deal.
7. After that scene, Book 2 returns to the college time frame and picks up the story where Itt has 30 days to make Pai accept him as a boyfriend. The Itt characterization in his section of the book is absolutely precious! As Pai himself comments, he never expected that such a "cute" version of Itt would even be possible.
8. At the end of the 30 days, Pai does accept Itt's love. Book 2 ends here.
9. Book 3 advances the timeline by 10 years and picks up with the group of friends in their various careers. None of this is in the series.