How is it? How does it compare to the first game?

Yes I've played it. I really enjoyed the first game and I think the new game is improved overall. It's bigger for a start. There are three maps you can play on and move to, each map being at least as large (probably larger) than the single map of the first game.

Gameplay is pretty much the same sort of thing, you prioritize survival and sustainability first and story progression second. You make your own quests based on what you need, just like before- rather than having canned quests like in other games. I liked the balance better. In the first game, once you set up outposts, the game was really easy. Outposts are much less powerful and you can't have as many. There's a wider variety of bases that you can move to, and they all have very different basic advantages- like one is very good if you want to maintain cars, one comes with a generator for which you only need to supply fuel for...etc.
Keeping and maintaining strong vehicles is much more important.
Otherwise the gameplay is very similar.

Graphics are better, but it's still a AA game made by a smaller studio and not a full on AAA game. But it's all serviceable. The strange time progression system from the first game, when you are logged out, is gone in the sequel.
Story is different, it's still pretty much optional to follow the story. This time it's simply about making a name for yourself- achieving a kind of social status on your map to leave a legacy to be remembered by. You can be a warlord, a law bringer etc. It could be better but at least it's a try at something different.

The bad side is still bad, it does get a bit samey after 15 hours or so. The game has a bit of a learning curve over the first hours, but once you have learnt it, it runs out of ideas- just like the first game. Most people will take a break after the first map, before coming back to the others later. Unfortunately reviewers compared it to big AAA blockbusters on PlayStation like Horizon Zero Dawn and rated it a bit unfairly IMHO. If in doubt it is on Game Pass, so as long as you're not afraid of using the Win 10 store (or an Xbox) you could use a Game Pass trial period to try out the full game for a few weeks.

I wonder if GOG is getting it?

Because its hitting Steam on March 13th. In fact, preorder is up for $29.99. Its called the Juggernaut Edition. It has a new map and updated/overhauled graphic too apparently and some QOL additions. Its the full game too (ie everything up to this point, plus more as stated)

I really doubt it. It's published by Microsoft. GOG has been getting games from MS studios, but only ones that already have existing publishing deals with other publishers like Deep Silver. State of Decay 2 is Microsoft all the way, so I don't see it coming here.
I haven't kept up with the add on stuff and updates, but as far as I know it's all been free to owners of the base original. I need to get back to it sometime to play the new stuff.