I can only give you some examples here. Some themes are not about same-sex love, but mixed with this element. As for greatness, I can't judge. You can read the book review yourself and decide for yourself.

Chen Sen's Classic of Flowers, Yu Dafu's Wide Night, Bai Xianyong's Evil Son, Zweig's emotional confusion.

Andre gide's Defectors, Wilde's The Picture of Dorian Gray, and Lawrence's Women in Love.

Brokeback Mountain, please call me your name.

Morris (there is a movie version, which is highly recommended. All three actors are very picky. Hugh Grant played the boy's inner struggle very well.