According to SteamDB and GOG DB, Steam version of Blair Witch
received an update in July 2020, after the last update in December 2019 which is still the latest update available on GOG. I couldn't find any information on the update but according to developers' comments in the Steam forums earlier that month, they were working on fixing bugs (so it's probably nothing Steam-specific). https://steamdb.info/app/1092660/patchnotes
Also according to those two sites, APE OUT
has been missing updates on GOG since 2019, with one 2019 and multiple 2022 updates being exclusive to Steam. It's also missing a macOS build. https://steamdb.info/app/447150/patchnotes
/ https://www.gogdb.org/product/1517967052#builds Firewatch
received updates in December 2021 which were only released on Steam. https://steamdb.info/app/383870/patchnotes
Can't confirm the Firewatch or Blair Witch updates being actual released updates or what they'd contain. SteamDB activity is hard to confirm if it meant anything really.
Ape Out will be added though.
Apparently Agony (original version not the unrated) is outdated. After finishing the unrated version I decided to take a look at the original just to see the changes ... but it was barely unplayable because it was too dark. I looked up some videos on youtube and notriced that it was much brighter in those so I looked at the patch notes here: https://agony.fandom.com/wiki/Version_History/Agony
As you can see: lightning has improved a few times and the difficulty settings have been changed. Also it is possible to adjust gamma using F7/F8 ... none of this exists in the GOG version. Since Agony has been released after the latest Steam patch it seems as if the devs released the unpatched version already at release - maybe it was a mistake. Already contacted them on discord and I will also create a supprt ticket (just for the funb of it since I don't expect anything from GOG support anymore tbh).
I'll try to update the list again sometime this month. It's hard to care to update when it doesn't feel like GOG itself cares.
I know how you feel and I had similar feelings yesterday regarding my German language patch thread ... but we should remember that we do this first and foremost for the community and not for GOG.
Did you receive a response from the devs?