Bought this one after watching some YouTube video which pointed out it's more of a Zelda-like and not a rogue-like. I will probably finish Plague Tale: Innocence first because I'm halfway through, but I will turn to this one soon.

GOG's got some good stuff today.

Wishlisted for now.

Does this game have leveling or is it fixed growth?

So far, I only played roughly 30 minutes of it, to try it out, and it doesn't seem to have levelling up, at least not in the RPG sense -- no stats to upgrade, no actual "level 1 > level 2 > level 3" progression; you do get to upgrade skills and weapons, though. It's more like a (traditional, pre-Breath of the Wild) Zelda than a "proper" RPG. From the bit I played, it feels like a mix of Titan Souls and Hyper Light Drifter but in 3D, instead of being pixel-art. The boss fights can be a bit on the hard side, but it never feels unfair, and it's most definitely not trying to be yet another Souls-like, which I find refreshing. If I had to pigeon-hole it, I'd call it a faux-isometric action-adventure ("with RPG elements", but, then again, which genre of game doesn't have RPG elements?).

[EDIT] Just adding that, like in Zelda games, you also get new weapons and items, even though you start with just a sword and a bow and arrows.

Is this related to that horror game that went viral several months ago about a developer who made game about his dead daughter or something?

The game page has the tag "achievements" but doesn't seem to have any. Is this yet another GOG mistake?

I don't think the mistake is on GOG's part, as this is not the first time Devolver Digital publishes a game in here claiming it has chievos when it doesn't. The Steam version does have achievements enabled, apparently, so my guess is that they will be coming to the DRM-free version soon.

True, Devolver doesn't seem at all reliable with their releases on GOG. Hopefully they'll add them soon and it won't be like Olija, which still has the "achievements" tag but in fact never had them.