So it is possible to check
1. If the files were downloaded correctly
2. If gog.com has changed the files (updates/patches)

For example see https://www.humblebundle.com/. Here you can find a MD5-Checksum for all downloadable files in your library.

1. If you want to be sure, download the game again using the GOG downloader and point it to the directory of the game's installer in question. It should verify it and if it has corrupted files, it should redownload them again. Alternatively, for single-part installers, you can do a digital signature check, while for multi-part installers, you can run them and click on the option to do a digital integrity check. In the case of the latter, the installer will tell you which of its parts is corrupted.

2. You get notifications, indicating which game has been updated. Unfortunately, the notification doesn't say what has been updated. You can check this thread to see what the update in question was all about.

GOG likes to change the ads in their installers from time to time. As such, published installer checksums would change frequently even if the game isn't receiving any updates. I suspect GOG is trying to avoid confusion.

It's too bad, I love the way humble does it.

If you're doing it, at least do it properly. :p MD5 is old and bad. Should go for SHA-2 instead.

No. For checking the file consistency, a crc32 (only 32 bit) or MD5 (128 bit) should be sufficient. This is not a crypto problem!

Game pages in the GOG wiki sometimes have them. I have no idea if they are up to date however.

Well gog installers do have an autotest function, but on large files it's pretty slow.

Isn't that only for multipart installers?

There have been some updates on the installers MD5, though not sure how up to date it is. Check [url=http://www.gogwiki.com/index.php?title=Template:Installerinfo&action=history]this page[/url] for last modifications. No. On multipart installers the self validation is found within the setup, on single files the digital signature is the self validation. If the signature is OK, then the file is OK as well.

I didn't know that. Thank you.

I do too. And I have written a userscript to finally implement this feature (gog.com/account):

https://openuserjs.org/scripts/xmanacollectorx/GOG_Checksum_Lookup

Seven years asleep, the poor thread was made to rise again.

And people wonder why I advocate for thread archival after a certain timing.

Or you could like you know.. ignore it if it doesn't help you. I however found this from google and found that xmanacollectorx's script is exactly something I have wanted for the past seven years if not more :P

Thanks for sharing your script, and while maybe it would have been better to post about it in a new thread of your own, just ignore those who like to get on the Necro Hobbyhorse.

Necro is only bad if it isn't pertinent to the thread. Some of us keep telling them that, but they just never listen. Just because a thread is years old, doesn't make it worthless or irrelevant. There are enough threads to search and wade through in the forum as it is, without needlessly creating new ones that are duplicates. And honestly, the search system here is crap anyway.

P.S. For the record, Humble having MD5 values is in my experience, useless. I reported a file that did not pass the MD5 check, and here we are over a year later, and they have likely still not done anything about it. I checked for several months and then gave up, and are still waiting for a notification from them that the files is fixed, as it was indeed corrupted, as confirmed by 7-Zip.