I'm a big fan of turn-based, RPGs where you walk through a grid maze. Some popular games like this would be the Etrian Odyssey series on DS/3DS, Wizardry, and Pool of Radiance. If you need graph paper to beat the game, I consider that a plus.

Among games on GOG, I've played Wizardry 6, 7, and 8, along with the gold box games.

Are there any hidden gems in this vein here on GOG?

(Note: I'm a Linux users, but up for anything if it works OK in Wine)

Grimrock 1 and 2 are the obvious ones. There's a steampunk style one too... Automata? Something like that.

Off GOG there is Might and Magic X, which is wonderful.

The Grimrocks are real-time though. Have you played the old Might and Magics (especially 3-5)?

I think Starcrawlers is turn-based, too.

Oh, and if you like Pool of Radiance you should definitely check out Unlimited Adventures, included in Forgotten Realms - The Archives: Collection Two (the same package that also has most of the goldbox games). It allows you to play tons of great user created adventures in the style of Pool of Radiance. (More info here)

I played most of the first Grimrock and enjoyed its atmosphere. It is real-time though, and I find myself doing stupid attack-backup-repeat stuff. And I kept forgetting spells. The puzzles were lots of run though.

I haven't seen a steampunk game in this genre. I don't see a "Automata" on GOG (or elsewhere). If you remember, please post the name. I've played M&M1. Maybe I'll give one of the others a try.

Apparently I already have Starcrawlers. I'll have to give that one a serious try. It looks like it brings some new ideas.

Elminage Gothic; it's a modern Japanese Wizardry-like, much like the Etrian Odyssey series, but more in common with early Wizardry, and (once you get past the first dungeon) harder and more brutal. I have been able to get the game to work in WINE (though I think I did need a winetrick or two). There is an automap, but viewing it requires using a spell or consumable item, so graph paper might still be a good idea.

Also, The Bard's Tale (2004) doesn't fit your criteria, but it does contain emulated versions of the classic Bard's Tale trilogy, which does. Also, I remember reading that Albion has, later in the game, a mandatory action minigame.

Of course, this reminds me that the early Ultima games (1-5 + Akalabeth) have first person dungeons; however, the dungeons are only a small part of those games, and Ultima 1 does have a mandatory action minigame later on. From what I understand, it is real time, but the developers added an option that makes time stop whenever you aren't doing anything, which can give the game a turn based feel (and make it accessible to those who can't handle real time).

I don't remember a minigame, but I think there are 2D/isometric levels with puzzles and traps that play a bit like Al-Qadim: The Genie's Curse or Blood Omen: Legacy of Kain. Maybe that's what you mean? They're rather easy though, you don't need to be good at action games to complete them. But yeah, it doesn't hurt to know that the game also has elements that are not first person and turn-based. Interesting, thanks for the info!

Oh, and there's also Paper Sorcerer.

If it has to be turn-based then I REALLY suggest Might and Magic X. It was super well done, a real surprise.

I got it on Steam and opened a Uplay account just to play it but never got it to work D:

The Fall of the Dungeon Guardians is also quite good

The OP is specifically asking for games on GOG though.

Thanks for all the suggestions. A lot of these look interesting. I might give the M&M 6 pack a try.

Elminage Gothic looks like fantastic. I love old Wizardry games, so this will be a non-brainer. It looks like it works well with Wine.

I'm now very addicted to Star Crawlers. It's so good. The only thing that would make it better is if it had terrible 90s style graphics, but you can't have everything :)

I think most of the ones of GOG have been mentioned except Dragon Wars and Bard's Tale.