I'm into VNs lately, pretty much all genres, although I hugely prefer anime-style drawing and voice-acted. But, having some trouble to sift through all of the offerings.

If you found a hidden gem of a VN here on GOG, could you share the title and why you loved it?

The Song of Saya, obviously. the nice and warm piece.
if it having voice acting isn't mandatory, Higurashi When They Cry series. abt friendship and trust.
there are some free ones that are not here. if you're interested, I could list 'em in PM.

If voice acting and GOG are not mandatory I highly recommend the Phoenix Wright games starting with the original trilogy.

There's some detective and court room stuff in them, but that are basically 70% visual novel.

Well, someone already hit Phoenix Wright, so that covers about 70% of what I was going to suggest so...Professor Layton is the 30% choice.

And I suppose for actual VNs, Katawa Shoujo. (Which I'm linking the Re-Engineered version of.)

in case of Katawa Shoujo I'd suggest going fur the original, though.

That depends. Are sexual Visual Novels your thing, wacky or serious, do you want gameplay, and so forth?

I really enjoyed The House in Fata Morgana.

The story is gripping and compelling, emotional and philosophical. Some adult themes, also. You won't easily forget this one after it ends.

I don't have any to meet your absolute preferences as I have my own, but perhaps if I enumerate some that are listed as Visual Novels that I personally enjoyed, at least one may have merit to you:
Available on GOG:
Disco Elysium
Astrologaster
Roadwarden
Chicken Police

Not available on GOG:
Who's Lilia

Not even available on PC...
Ace Attorney Investigations: Miles Edgeworth

Not voiced, but imo is a hidden gem: https://www.gog.com/en/game/sound_of_drop_fall_into_poison

Gameplay if fine if it's turn-based. Sexual, it's okay if there are H scenes, but I'm not looking for pure H games.
Both serious and wacky are fine.

Some of my favourite VNs / VN-like games are: White Album 2, House In Fata Morgana, Raging Loop, Aokano 4 Rhythms, Paranormasight, AI Somnium Files, Danganronpa, Every Hue Of You, Highway Blossoms, Stein's Gate, Until Then.

Disclaimer: I usually don’t like Visual Novels, at all.
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If you have not tried it yet, I very strongly suggest the very best VN I know of: Slay the Princess.

I wrote about it a bit, in French, on La Couronne de Cuivre forum: [Visual Novel] Slay the Princess.

It comes with full voice-acting, nice graphics (in a distinctive style), ambient music that strengthen the mood, and is built in such a way that it has very high replayability value.
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A couple other visual novels I recommend:
- Eliza
- Werewolf: The Apocalypse - Heart of the Forest
- Vampire: The Masquerade - Shadows of New York
- Coffee Talk