Man, I had to look this up because I saw the question and realized there's some confusion floating around. 'Gen Y' the series is actually a Thai BL show, a sequel to 'Love Sick: The Series'. It's fiction, a continuation of that story with the same actors. I think the mix-up comes from the title maybe sounding like a documentary or something about millennials? But no, it's pure drama – all the school rivalries, friend groups, and romantic entanglements are scripted.
That said, you could argue it's 'based on' the real-life experience of being a teenager in that kind of intense, friendship-driven environment, but that's super broad. The characters and specific plotlines, like Phun and Noh's journey or the new couples they introduce, are original to the novels by Indigo. So, real-life events? Not in any direct, biographical sense. It's just good, addictive story-making that feels real because the emotions are relatable.