I'm trying to understand how installer and patches work together and I have a few questions for GOG.
1. Is there a common way for all GOG games to handle patches or is it case by case for each individual game ?
2. Patch is by definition a delta change but compared to what? Previous full installer? Latest patch ?
3. Do I always have the latest version when I load an installer or should I also load patches ?
4. Is it possible to have multiple patches? Should I load and apply sequentially all of them ?
5. How does GOG know that there is something new for me in my library? Based on what criteria ?
6. Is there a RTFM I haven't read?

Any clarification appreciated here.

normally, you should have a base game and a patch to download. First, install the game, then apply the patch.

If a patch comes out of a game you have already installed, just install the patch.

Eventually, game and patches get bundled on a new base game installer. If you want to update your game to that new base, it can get tricky, but normally reinstalling the game over the old version works.

What P1na wrote. You have an installer. And maybe a patch. Current full installer - I think. If there's a patch, that one is the latest version. Otherwise it's the current installer. I think I've seen it once. When in doubt, run them in order - they should include a check if they can be applied and give you an error otherwise. The games get "flagged" manually (i.e. by Judas) and thus notifications for all people owning the game are created. The notification will (should) go away if you click on the game in your account (like when you would download it).

The system is far from perfect - you will get the notification if the Mac version is updated, even if all you ever use is the Windows version. Sometimes games will get flagged as updated repeatedly (DragonSphere anyone?) by mistake/bug.

GOG Galaxy will change the update process for most people (unless you choose not to use it) since it will probably auto-update your games (just like Steamd does). I've seen it working in AvP 2000/Galaxy Beta (or rather Alpha) Test and if looked pretty good.

Although there may be instances where the patch is older than or the same version as the installer. If that is the case, the version check will report that the patch has already been applied. In these cases, the patch is for the previous version of the installer, and was left up so those who downloaded that can update without downloading the installer again.

As the others have said, installer then patch.

I remember someone saying that on Gog you can choose which patch level to apply - this made me think most games keep their old patches and original installers - so you could use any version (from newest to original release) as you please.
I guess from the answers to this thread I was wrong? I never got into checking as most of my games are rather old and don't get patches.