It's probably the latest 'Tech Update' patch, the list of changes can be seen i.e. in the Wiki. This patch came to GOG on 2021-11-26, so one day after Steam.

We're still missing 1.6.2 patch, which came out on Steam a couple of weeks ago.

Thanks for the answer. But the last update that I can see on the Wiki is the one surfer1260 posted about'1.6.1_51795_59550' (Tech Update [25/11/2020]).

But thanks to the steamdb.info (I'd forgotten that site existed!) link you included, we can see that there's been 7 updates since then (including Patch 1.6.2). My guess (based, entirely, on the fact that some of my game files have been changed around the end of November 2021) is that our latest update is: https://steamdb.info/patchnotes/7179920/ which was released to Steam 12 August 2021.

If that's the case, we're (hopefully, at worst) about three updates (including 1.6.2) behind Steam.
Or at best (if those changes were to include advertisement for 'South of the Circle') a building (Builders’ Monument) and 1.6.2 behind Steam.

My guess is based on that SteamDB show that files have been changed, but nothing has been mentioned it, in the Patch notes (except for adding the building and 1.6.2).
Let's hope we get a new update before January! :D

Yes, sorry for my mistake with the dates - GOG's patch came out a year and a day later than it appeared on Steam, it seems. Anyway, the patches don't contain big changes IMHO, but maybe it's worth asking on their Discord about the GOG release.

According to what a dev said on their discord from a few days ago

the game will be EVENTUALLY brought up to current steam version the dev claims they are busy with FP2 and other announced but yet not out games.