Are all GOG games up to date with the current game version and all of the official patches?

Do you mean the GOG patches, or the patches out there in the internet? Either way it's a broad statement. If a GOG game has GOG patches, the installer is NOT patched. Nevertheless, some games from before the era of GOG patches are patched instead, because they had no other way of distributing the patches

As for patches out on the internet, who can say? Often yes. Sometimes no, because GOG introduced fixes that aren't compatible with a patch, (sometimes because GOG's fixes were better for modern machines) but that's rare. Sometimes no, just because. Fan patches are included more or less never, but I think Dungeon Keeper 2's GOG Patch involves similar fixes to a fan patch and for all I know might include it.

Sometimes the games' description will mention a patch, but not always.

The situations can be so varied you'd be better off asking game-for-game in the respective forums, or searching the forum for posts on the subject.

In 99,9% of the cases yes. All official patches are applied. Obviously fan-patches can't be applied without the authors consent and getting hold of these is sometimes impossible.

Kings' Bounty, iirc, is the only game not up to date, as the expansions are missing or something like that. Others might know more.

Umm, AFAIK KB had no expansions. Armored Princess is a standalone game, not an expansion.

I don't know what exactly was wrong, but there were some issues with that game (unless I mix it up).

Some of the games have fan patches applied too, like Gothic 3 Forsaken Bugs for example.

I mean the games official patches.

The full installer might be patched even if GOG also offers a separate patch. In these cases, the separate patch is for those who downloaded the full installer earlier and would rather not download the whole game again.

Example: Wallace & Gromit's Grand Adventures. Of course, this is an official patch, but it was only needed for the GOG version of the game, the other versions did not have that bug.

Enhanced edition/Gothic 3 Community Patch , while made by community, is officially approved patch for the game (both G3 and forsaken bugs :-p). Original War devs also outsourced patching of their game and they only do QA and approve them for release. These, unlike most fan patches, also have access to game source code.

There are at least couple games on GOG that I know of that have been patched with unofficial patches (Warlords Battlecry 3 and Master of Orion 2) and I'm sure gog also uses number of individual fan fixes for specific game problems.

Indeed they do. The Quest for Glory games are an example here.

So in general, GOG games are reasonably up to date, and I think the rule of thumb is that 1) all official patches are always applied and 2) if there are any bug-fixing fan patches, they are also applied. But things like that one massive Fallout 2 patch or the numerous improvements to early Ultima games aren't used for purism reasons.

And yeah, having consent of the patch's author is another issue, but some of the patches are basically abandonware by now, so this is a pretty fuzzy area.

GOG is using no CD-cracks quite liberally regardless of origin. But the authors of those will hardly complain ;-).

The case with fan patchers might be similar to crack makers. They are interfering with the source material without right owner's consent in the first place, so they don't have any claim before court to their work.

The Septerra core "patch" fixes the freezing after battles, but loses the loot info messages. Plus it's installed like malware, as i cannot find where it has been installed, there's no uninstall info, and when i have un-installed the game and re-installed, the patch is still there.

Grrr! I hope GOG support don't recommend this patch any more.

Of course they do. The crackers also have claims, but the right holder can pretty much sweep those up with the same proceedings. With "fan patches" it is much more complicated, as they feature a variety of changes which can cause serious trouble for GOG.

Not worse the risk, and that is all that GOG will ask their lawyers.