I seem to remember that when a game was updated there was a flag next to the updated file in your game library so you tell if it was the game itself that was updated, or simply that new 'stuff' was available - like music, play guides etc. . This doesn't seem to be the case (now). Is there a way of finding out what an update is?

Most of the time there is a changelog where you can read what was done. Under "More" -> "Changelog" when you click on a game.

Aside from that, you can only find out by following this thread:
https://www.gog.com/forum/general/the_what_did_just_update_thread

yeah that's what I thought. I was hoping there was something simple, like a flag to indicate which files were new or updated..

Dunno, doing a quick skim of the changelog seems pretty simple to me. If, for example, it only mentions compatibility fixes in the OSX installer, then I (as a Windows user) know I don't have to bother re-downloading the game.

Not all GOG games have a changelog.

Anyway, since I occasionally run gogrepo.py to keep my local GOG game collection up to date, I don't care to check much what exactly has changed (and whether I should download it). gogrepo will automatically download all changed or new files relevant for me.

Unlimited data quota and fast broadband is nice, right?

In the library there is a pull down menu on the left side just above the games : MY COLLECTION (000) V Click on that down arrow to choose to show only updated or only hidden games.

Just a basic 10Mbps cable modem connection that is included into the basic housing costs by default. I could upgrade it to a 100Mbps cable modem connection, but I don't feel like paying the extra fee every month for extra speed I'd need only occasionally. This will have to do for now.

My wife's phone subscription got upgraded to 4G, and it is actually considerably faster than my current (fixed) cable modem. :) It is also supposed to be unlimited, but I presume there are some clauses for very heavy usage, ie. it might be slowed down if you max it out constantly or something (ie. exceptional usage)... Yeah... but sometimes GOG seems to do some silent updates as well which are not announced. Plus, to me it appears sometimes the notifications disappear also when you e.g. merely go to check the said changelog.

And as said, not all games even have a changelog, so you don't know exactly what has changed.

What I'd like is a feature I'm pretty sure used to exist; an indication which is the new or updated file. Let's say you've been notified 'Game X' has been updated. Is it a new avatar file, a play guide, or an updated setup Game X file?

I'm pretty sure it used to say "Updated" next to the relevant file.

they dont have that

A change of heart?

Yeah, I think you might be right -- but that was long, long ago.... :)