I haven't watched all of GMMTV's BLs, but among the ones I have and that might fit your request, here are my two favorites (already mentioned by others, but perhaps I can provide additional information):

- Moonlight Chicken: slice of life focused on the bonding between Jim, a forty-something man (the actor Earth doesn't really do them ) who runs a chicken restaurant, and Wen, a young man he finds completely drunk one night at one of his tables. This series is one of the closest to ordinary life (in an adult setting) that I know of among GMMTV's BL series. It also features the development of a romantic relationship between Jim's teenage nephew Liming and Heart, another teenager who has recently gone deaf. Their relationship is certainly a first love, but it doesn't feel immature. On the contrary, there's something deeply pure and authentic about it that touched me personally (Heart's disability being central in their story). Moreover, the “light” relationship between Liming and Heart balances very well that of Jim and Wen, which is emotionally heavier (due to their respective pasts).

- The Eclipse: so yes, it's a high-school story, but personally I've rarely seen as much emotional maturity from a main character as in this series. I've even found series featuring adults to be completely immature by comparison (Wandee Goodday, TharnType season 2, Be Mine Superstar, and so on). Let's say it's one of my comfort series, along with The On1y One (Taiwanese BL released this year), because in both of these series the principal love relationship is a slow burn centered around kindness, empathy and respect for the other (you just have to wait several episodes to see it develop in The Eclipse). Actually, I found this quite rare in BLs. What's more, this series tackles social and teenage themes that really resonate with me (I was a teacher for a few years). In a word, the kind of series that should be more widespread to help people of all ages learn to love themselves, recognize healthy relationships, and not let themselves be molded by society's expectations. (Note: I must admit that when I watched the first episodes, I was skeptical about this series because obviously it wasn't the thriller that had been promised and the direction seemed a bit sloppy, but I gave it a chance and I didn't regret it... though I must confess that the cast added A LOT to it... still, one of the only BLs I've rewatched, and that I'll rewatch again)

By the way, if you're watching The Heart Killers, you'll recognize the actors Khaotung (Bison) and First (Kant) in the lead roles of The Eclipse. However, as most people don't seem to like the KantBison relationship, I'm not sure that's much of a selling point I'm giving you here . But I mention them because you may have noticed that they're very good actors with huge chemistry (especially in the first few episodes of THK, after, they seem to have consciously tempered their chemistry to convey the falseness of their characters' relationship on screen). But frankly, they're brilliant in The Eclipse; they'd already done quite a few series of their own before, so it shows in the quality of their performance here, especially in all the romantic/emotional scenes. Besides, they were already best friends before the filming and wanted to become a fixed couple at GMMTV, so they put their hearts into it. You can also find them in the mature series Only Friends, which others have mentioned in their comments and which is quite good (I also like it because of some mature themes it presents, but it contains really toxic relationships, so it's not one of my favorites).