Hi, I'm new to GOG and I just came here to buy "What remains of Edith Finch" but I see that there are a lot of interesting games, I don't usually play classic or strategy games but I really like good stories (I love reading classic novels and science fiction), what would you consider is an interesting game for its storytelling? I don't mind about the genre or the release year, I just want to check some recommendations :)

Thank you.

Try Where the Water Tastes like Wine. The game is FULL of good stories.

Planescape: Torment

Thank you! :)
Any good series that I'm missing and I should start with? I'm seeing a lot of "Ultima" games, also Dragon Age and Wizardry but I'm not sure which one is worth as a first buy.

Hellblade: Senua's sacrifice (an interesting mind trip)

Far Cry 5 (purely for the story)

There are literally thousands of games we could recommend some of which you may not like the best thing to do is research and see which ones grab your attention.

Dragon age origins was great, dragon age 2 not so much, it branched off from the original concept and made it hard to like the game completely, Dragon age inquisition tied them all together, but still not as good as origins.

as to Ultima and Wizardry, it's personal preference, I personaly do not think they aged well so I don't play them, but that is just me.

Half-Life

Triple recommendation. No game I played can even get close to it in terms of storytelling (not even the Witcher series), and I played hundreds. Maybe even a thousand.

Since you like science fiction, I'd recommend Gemini Rue. The story and especially the characters left a huge impression on me.

The Witcher has some good story to it as a series. The Book of Unwritten Tales series is good. And Baldur's Gate.

Ultima is a good start. I would suggest starting with Ultima 7 (and use Exult to improve the graphics). And then, if you like it, play Ultima 5 and 6 next. Both are also story-intense, but with very dated graphics.

You could also give The Witcher a try. I love the world and the overall story of Geralt. 3 is the most polished one and is a very good role playing game. The two DLCs each add a different kind of storytelling to the main story.
Withcer 1 had it's quirks, but I still enjoyed it. Witcher 2 was amazing for it's time as well.

Unforeseen Incidents (adventure) - I wrote a rant here. Merciful (no premature game overs).

Primordia (adventure) - I wrote a review here. Merciful.

King's Quest 6 (adventure) - hardcore fairytale adventure, hard but fair puzzles, you can die, some note-taking recommended. The pack also contains KQ4 and 5, which are merely okay. 6 is awesome.

Hatoful Boyfriend (visual novel) - a completely batshit visual novel that parodies the dating sim genre (you don't need to have played a dating sim to get the jokes, and the jokes are not why it's awesome). Merciful-ish. (Holiday Star is fanfiction-grade bad.)

Another World - unfair platformer with an awesome wordless story. Short but annoying, has an easy mode which is less annoying.

Arcanum - the best RPG on GOG, possibly ever. ( Anachronox is also awesome, but Arcanum aged better.)

Most adventure games have great stories.
I recommend you have a browse through the Adventure section, and see what takes your fancy. ( :

For games with good stories that are here on GOG, I'd recommend Shadowrun Returns and it's DLC. The first campaign is really good but the Dragonfall DLC has phenomenal writing. I also find that Syberia and Syberia 2 have a nice charming story. Fallout: New Vegas has a lot of neat characters and some good writing with it's sidequests. Dragon Age: Origins also has great writing/characters. Telltale's The Walking Dead is another excellent story, especially if you like making tough decisions that directly affect the characters themselves.

For games outside GOG, I would also recommend the Mass Effect series along with Dragon Age 2 because they're some of my favourite games of all time and they have such great characters and stories. And if you enjoy Telltale's games but wish they had more time-travel and emotional devastation, then you should check out Life is Strange. Hope this all helps!