It has even two: 2070 and 2205.

SimCity 3000 Unlimited on its own is considered easier than SimCity 4. However you can make the game even easier by pressing CTRL + ALT + SHIFT + C to bring up the dialog box, then type in "i am weak" (without quotes.) You can now build anything for free.

Dorfromantik is a game about building idyllic village landscapes. You can't control what tile shows up next in the pile in classic mode (which is the main mode to play if you want to unlock tiles and biomes), but it's still a very relaxing game.

ISLANDERS is a game about building cities on islands. You can't control 100% what type of buildings you're placing as it gives you options, but it's still a really relaxing game.

Kingdoms and Castles is a game that I don't know much about except that it has a fantasy setting and was one of the games that inspired ISLANDERS.

Well now I'm doubly disappointed that neither are here on GOG.

May not be the best, but I believe this is the chilliest Sandbox available here:

https://www.gog.com/game/townscaper

You can't run out of money, because there is no money. You never run into problems, because there aren't any... Well, it looks cool and you can export the OBJ file to a 3D tool. (Nothing much besides that)

As a game - it is very cool!
As a city-builder... it depends... there are 2 types of threats in this game.

1.) Vikings who come to either steal your gold or kidnap some of your villagers.
2.) A dragon that likes to come by every few years and show you a ''cool'' fire trick.

The vikings - the first 2-3 raids you have to endure. :/
After that... at-least in my playthroughs, it was no problem to repel them and I was awaiting them to land.

The dragon - managed to shoot it off before it 1st appears.

While not terrible, these 2 threats might make the ''chill'' the OP is looking for non-existent.

Also - since I last played it, the game has been substantially updated. There are now AI neighbors. It's also possible you can turn off the dragons and the raids. Not sure about this. Probably not. =P

---

To OP:
Infraspace is not exactly a city-builder, but more of a logistics managment. Similar to Factory town.

The entire idea of the game is to have enough water before the drought hits! xD
A fantastic game... but not the ''chill'' you are looking for probably.

Others have mentioned many good games, such as Fabledom -- yeah, I'd also like it to come to GoG. :/

Whilst I haven't played it myself, so can't tell how ''chill'' it is - you might want to take a look at Children of the Nile.

One user, ElSketcherino, put this in a review:

btw, team 17 also put out quite the chill fun city builder out there

Aven Colony

It is even nice to follow the campaign. On its most difficult settings the goals are still pretty much in reach of anyone i think :)

The threats would definitely be a factor in terms of how chill the game is. I mentioned SimCity 3000 primarily because you do have the option to turn off disasters and just not deal with them (on top of the everything is free cheat.)

Some games can be situationally relaxing. Civilization 4 can be forced into being a relaxing game if you're playing a large map on the easiest difficulty with only one opponent.

Find the free/open source implementation of Fracas. It was a "risk" clone with some sea mechanics added. Super light and stuff... But with the map generation, and various AI settings, I loved how some of the maps would evolve over play.

Urbek City Builder might work for you. It's a bit unusual, in that you only place special buildings and regions for housing, and the city condenses around those special buildings according to various criteria - but it works well, and creates a really quite lovely experience. It's a bit of a puzzle, and you can shoot yourself in the foot a little bit by using up too many of its many resources and breaking conditions which you previously met - but even then, none of that is really permanent, and you can just puzzle your way back to a healthy state.

Sorry for the late reply -- I didn't see your reply till earlier this week. When I did, I decided to do for this game something I've long thought about doing for a handful of games: I wrote up a bunch of tips (which is why it took me another couple days to respond to your reply :) ). You can find the resulting wall of text here, if you do decide to give it another try.

No worries, life goes on ! Thanks for the text wall, I will surely read it and see how it goes on my next attempt !