Every day around the same time I'm regularly checking out the 'upcoming' list of games on the main page.
However quite often i see new, unknown to me games being released without them being in the upcoming list before.
So how does it exactly work?
Are some/most games not included in that feature?

Indeed. Those are the "surprise" releases.

For various reasons, they can't/don't want to announce everything ahead of time. This applies especially to old games, as well as most other more recent games that have been out for a while elsewhere.

Basically, the Upcoming tab is not comprehensive, but if something's on there, it's almost guaranteed (almost being the key word) to get released here -- if not on launch, then at least within a couple weeks thereafter.

Oh, no. Look another feature on the GOG site doesn't work as intended. Gasp, shock and general disbelief.

...
...
...
...
...
...
...
...
I died from the shock, but have since revived. >.>

I don't think this qualifies as 'not working as intended'. They don't announce every game they've got in the queue for release. The do save some back for surprises.

*edit* And I also wouldn't be surprised if some are finally secured for sale at an 11th hour, if you will, so they don't announce it's coming ahead of time.

Isn't that like over 90% of releases here? It's not a surprise if you frequent the "unofficially announced for release" thread though which usually only the forum regulars tend to see anyway.

My guess: The games there are the new releases; games currently created and they already have all the contractual boring stuff done and each party involved is working towards a gog release. The classic launch announcement.

The "surprise releases" are games gog is working towards getting on board which already are done and have been released in one form or another; sometimes even decades ago. They are not in the upcoming list because there is nothing sure to announce. They are either still trying to make the game compatible to modern systems, and/or still negotiating with the owners. The moment the negotiations are successful and the retrofitting of the game to modern systems works for sure there is not much to announce. They just release on the next possible release day.

It sure would be interesting to have a "games being negotiated / being made compatible" section but that would make ppl just moan & complain as soon as something gets delayed, negotiations fell flat on the face or just cancelled. For us its a surprise release; for gog there is not really an option. Unless they want to stall the release themselves; but why should they?

If. Not everyone frequents the forum, much less that specific thread. Plus, even if you do:
1) Release date is not always known.
2) The release itself is also not always guaranteed.