I struggle to understand if I should pick the patch or the installer?
Is the patch incrementally applicable over any version I already have and the installer is like the full thing?

Patch usually only applies to the previous installer. The installer is the full thing, and has been updated to the latest version.

Thanks for your answer. Problem is how can I be sure I didn't skip an update? Does GOG keep all history of all patches? Should I download all of them? I guess this question is not limited to this game, wonder if GOG has a generic way to approach it.
Still very confused about the use of patches.

Edit: just extended the question on the general forum, this is not specific to this game
http://www.gog.com/forum/general/question_about_installer_patch

If you can't apply the patch, it will inform you when you try to patch the game. I know this patch will install over the monster guide update but I haven't tried it for earlier versions. I'd personally just download the installer and use that since it isn't a large game.