The list yes, the number of games that phone home ... maybe not so much. The creator of the list will certainly not be able to test every game available on GOG and additionally it may be hard to detect if a game phones home beyond what is maybe regarded as harmless (looking for updates...).

One could see it as the lower bound.

Thanks for the many useful tips in your post. I will try some. However there is no way I will physically unplug the network cable before I start a game on my computer (too much crawling under a desk involved). Writing batch files that disable/enable network adaptors might be a rather simple solution, but I would definitely prefer a firewall. Will try to learn more about it.

It should be as low amount of hassle as possible but I also want to keep my usage data to myself as much as possible. So a compromise is needed.

Sorry for the resurrection, but this topic is now more relevant than ever!
I think the OPs link directed to the GoGmix that listed games with spyware, but gog decided to get rid of GoGmixs soooo... THANKS A LOT GOG FOR YOUR LACK OF TRANSPARENCY!

We know that:
Sony/PlayStation PC LLC games have spyware/abusive EULA (Horizon Zero Dawn, UNCHARTED, Days Gone etc.)
and
the vast majority Unity Engine games, the so called unity analytics spyware and sometimes a "facebook module".
Everspace used to have Redshell, but it was apparently removed
Kerbal Space Program & Vaporum also contained Redshell (removed on steam, gog situation unknown) and who knows how many other games.
XCOM: Chimera Squad is a major offender containing 7 different analytic tools/or spyware!

To name a few, and there's probably more cases like this among GoG games!

Anyway, i would like to know if the gog version of BioShock 1(according to steam the version is unchanged from 2013 and safe, but IDK about the GoG version) & 2 classic and Mafia II classic & Mafia III are spyware-free? The reviews on steam are going berserk because of the new 2K/Take-Two abusive EULA.
I know for a fact Civilization IV does not have spyware. Not 100% about XCOM: Enemy Unknown, but the last time i checked the gog version was clean.
So, are BioShock 1&2 and the Mafia trilogy clean?

Im not condoning any bad behavior from companies, but you can play 99.9% of gog games offline. Callouts wont matter.

If anything exists in the games, it wont stop the attempt. But the games have offline installers for a reason.

At this point is better to assume every game use telemetry one way or another.

Running a offline machine might not be the most pragmatic solution so firewall block may be the best way.

Edit: is there a simple to use, decent firewall alternative for Win10 and above?
I used TinyWall a few years (10?) back on a Windows7 machine but there was some conflicts and the program crashed regulary, soon after I moved to win8.1 and never installed it again, not sure if still is a thing.

Edit n2: I wouldn't mind some sort of launcher similar to Bottles and Lutris on Linux, specially Bottles (via flatseal). Self contained and with a simple on-off switch for online connectivity.

I'd say ZoneAlarm but now they've added some AI crap (because of course they did!). Older versions are no longer supported and (having chatted with their support) they don't really seem to know what the AI does other than "protect you - with the power of AI!" - which says to me its as trustworthy as letting the telemetry get online.
I think its also no longer available as seperate firewall (all the links I tried including those from their support include the AV as well).

Windows Firewall Control works great for me.

Kinda have an aversion to Anti Virus, thanks for the sugestion though.

Will have a look, thank you.

Just tested BioShock 2 and yes the EULA was updated, but there was no spyware to be found (or maybe it was just well hidden). However the game was a totally unplayable crash-fest.

And found 2 more games with potential telemetry/spyware:

Xcom 2
https://www.gog.com/forum/xcom_2/xcom_2_privacy_policy_warning_for_could_be_potential_playersmany_do_not_read_these_lengthy_things

and

Deus Ex: Human Revolution - Director’s Cut
https://www.gog.com/forum/deus_ex_human_revolution_directors_cut/phone_home

Thanks for the reminder as to why I whitelist internet access to all applications, and why I don't necessarily automatically update things, especially when they are currently working fine and are no longer receiving actual updates.
But yes, XCom2 does appear to have telemetry or some sort in it. Without having to dig much further, I just opened it and good old TinyWall blocked an attempt it made at an outbound connection. If I had owned this game before the telemetry patch, I would have utilised a saved a back up of the previous installer.

Yes, Unity phones home and if you want privacy, you need to block it. That is well known.

Yes, yes, you're right of course. I just didn't realise that it was normal for UE3 to have telemetry in it.