Not to mention the missing Mac version on GOG.

Somebody wrote a script that collects the discrepancies: https://airtable.com/shrldLsErlUf3eHqS/tbltXjS8fxEGG11eD
There are some false positives (I can confirm that Curse of the Dead Gods is probably just a typo) and it only collects once a week, so some of the entries might be out of date, but it seems to work fine.

Thought WinterSnowfall was tracking?

So then there's such thread already?
Care to share a link to it then?

EDIT:

I took a quick look at this list and it doesn't cut to the chase.
It only tackles one of the issues that the supposed thread to be created would follow.
This list only really shows if some given game has outdated offline installer when compared to Galaxy-originated builds.
It does NOT show for example if there's a discrepancy between what version transitions offline patches cover vs the available number of builds total - I can name one specific example in regards to this where offline patches only cover a subset of builds and so every few versions a full offline installer based reinstall is required in order to up the version - this creates a disadvantage for offline installer users (as naturally Galaxy does not have this problem) while the list does not cover such occurences at all.
Another example is that for quite some games DLC offline installers are only available for the very latest version of the game, that combined with the previous outlined problem creates a predicament in which under certain circumstances you would have to do a full offline reinstall just to install DLC.

Not a thread, but that list soosgjr linked to. Dug up where it was announced, and see the posts above that too.
Yeah, that's just to compare the situations where the offline installer really is behind. Pre-Galaxy GOG did not tend to offer patches (admittedly, for old GOG there wasn't that much need), so I for one don't consider not having separate offline patch files as having worsened anything. Or, in other words, Galaxy users get an added benefit, but offline installer users don't lose something they used to have.
The problem is this trend of stores getting separate versions and handling patching themselves, instead of games being able to either update on their own (if allowed to connect to check, on a manual request) or patch files being generic and downloadable off the game's / developer's / publisher's official site.

The thing is:
1.Neglecting patches for "pre Galaxy" games is still wrong
2.I have a very specific example for a game that is fairly new and still being updated.

https://www.gogdb.org/product/1957528513#downloads
https://www.gogdb.org/product/1957528513#builds
https://www.gogdb.org/product/1957528513#changelog
Notice how offline patches don't span the entire build range and every few versions you would have to download the full installer. And this is a rather large sized game so...
You can even see one patch being removed (just... WHY) in changelog.

Also, the DLC installer seems to be only available for the very latest version of the game.
Combine it with the previous notice and you got a predicament combo.

*list puts on sad face emoji* :P. It is what it is.

I get the problem here, but to be honest it was never meant to track this (more nuanced) issue in particular. And given the consistency of the entries reported by the GOG APIs, compiling such a list programmatically would be akin to trying to shoot yourself in the leg... Unfortunately I don't have the time to look into this at the moment, even if it were more pleasant a task. In fact, my GOG surveillance server is currently down (due to hardware failure), so expect the list to lag behind more than usual for a while... *sigh*.

Can aynone confirm if Nanotale - Typing Chronicle is out of date?
On Steam it received an update two weeks ago introducing a better map etc, but if I check gogdb it seems like the last update here was in April.
I'm on the fence of buying it, but if it is out of date...

This games where upated?

Not yet - we probably might need a bit patience.

Foregone (Big Blue Bubble / Big Blue Bubble) is missing the paid soundtrack DLC on GOG (and on the EGS, curiously).

Also, SteamDB lists the original build released on 22 February 2021 and one more build on 4 March 2021. GOGDB lists only one build released on 23 February 2021.
It looks like the Steam update on 4 March 2021 removed a file called "Foregone_steam.exe", while also changing some asset files. It's possible this patch never got to GOG because it only addresses problems specific to the Steam version, but that's just conjecture at this point. However, since the game was an EGS exclusive before coming to Steam and GOG, it's reasonable to assume that the GOG build includes all patches and content updates released for the game over its EGS exclusivity period, so it should be fairly up-to-date.

Oh, ok...
PATIENCE

GRIP is missing online multiplayer and a bunch of DLC packs; Artifex Car Pack, Delorean 2650, as well as the free Artbook and Manual DLC. It tries to make up for this by having a lower base price.

The cars could be licensing issues Art book unsure maybe the same if it has cars in it.. but the manual yeah no reason why that wasn't included unless maybe it has images of cars that again has licensing issues.. Multiplayer That might have to deal with servers if its that sort of multiplayer.. But meh Multiplayer for me is nothing I care about anyway if its online multiplayer..