Can anyone knowing titles made by Ukraininan studios share some titles, which are available on GoG (preferably decent)?

GSC Game World
Cossacks and S.T.A.L.K.E.R. games

I believe the first two Metro games were made in Ukraine but the studio, 4A, relocated to Malta in 2014. Many of the same people still worked there but the composition of the company is more generally European now, as is my understanding.

Best Way, who worked on the original Men of War 2009 game (as opposed to it being the second or so Behind Enemy Lines game) is also Ukrainian whereas the studio which helped them, Digitalmindsoft, is German. I suppose you may find it curious to know that publisher 1C Entertainment, last I checked, was based in Moscow. May or may not be of note. Best Way also worked on both Faces of War and Soldiers: Heroes of World War II.

It is not really related but Codename Panzers Phases One and Two were both made by defunct Hungarian studio Storm Region.

That was what I could dig up from my library (aside from STALKER and Cossacks of course. Although, GSC also made Venom: Codename Outbreak and American Conquest).
It may not be relevant but GSC is listed as publishing their games on here.

Frogwares - https://www.gog.com/games?query=frogwares

edit: also this -
[url=https://en.wikipedia.org/wiki/Category:Video_games_developed_in_Ukraine]https://en.wikipedia.org/wiki/Category:Video_games_developed_in_Ukraine[/url]

Great!
Metro and the new Sherlock Holmes games were on my wishlist anyway. And of course I should have guessed that S.T.A.L.K.E.R is Ukrainian (I forgot about Cossacks).

A little summ up:

GSC Game World
Frogwares

I could recommend:

S.T.A.L.K.E.R series
Sherlock Holmes versus Jack the Ripper

The very best? Men of War series of WWII/Cold War tactics. Cossacks strategies, the first is the pinnacle, subsequent games aren't even half as good, but also worth a purchase if you like historical strategies (and might be worth it to check their American Conquest and Heroes of Annihilated Empires as well).
S.T.A.L.K.E.R. from the same studio that brought us Cossacks, and Metro are great post-apocalyptic survival shooter series.
Frogwares' Sherlock Holmes and other adventure games are pretty decent.

Of the rest, Collapse was critically panned for being a crude DMC knock-off, but I don't regret buying it (deluxe metal case, even), and neither I do regret buying You Are Empty, which also was objectively bad game that I liked a lot for some reason (aesthetics?). I don't think it makes things any better for people who care about that sort of stuff, especially since Tencent owns them whole, as opposed to mere 40% they have in Epic. They also have been known in the past for more reproach-worthy things like draconian (and buggy to the point of making games unplayable) DRM, unsavoury business practices and occasionally not paying developers when they could get away with it. And really shitty code when it came to their own products.

But they also published some great games that I'm not sure would've ever seen the light of the day otherwise, helped development teams in Russia, Ukraine and altogether outside of former eastern block to find their feet and lately they bring us some interesting pseudo-indies with a retro feel. And they release their games on GOG, DRM-free and stable for the most part, which more than makes up for their past transgressions, I reckon.

I looked again and it came back mixed. Their European site said they were based in Prague with offices on Moscow, Warsaw, Prague, Gdansk, and Budapest. Gematsu said they were based in Prague, but that was once again for their European division. I could not find a financial statement from them any newer than 2017 so, lacking any evidence to the contrary, I will concede to you.

Kind of disappointed Tencent purchased another company I used to like. The moral of the story in sum is that their Wikipedia page is woefully outdated.

Cossacks 1 is one of my favourite games of all time.

Ostriv is a nice sim game, I recommend it.