I've downloaded a bunch of offline installers but I don't really know how to verify the integrity of those files apart from using the option during installation. Is there like a way to get MD5/SHA256 hash values for me to compare against these files? Are there any tools to automate this?

github.com/hippie68/gogcheck

gogcheck

Bash script for the purpose of scanning your GOG offline installer collection for valid digital signatures and correct checksums, making sure your downloads have not been modified by someone else.

I searched all over GOGDB but I didn't find any list containing hash values. Where exactly can I get it? I checked lgogdownloader (can be installed via my package manager) but it doesn't specifically mention about verifying the integrity of an installer. Are you talking about the "--repair" option?

It's Github rather than GOGDB. The "AIO" looks to be an All-in-One list of all games tested:-
https://raw.githubusercontent.com/GOG-Games-com/MD5-for-GOG.com/main/AiO.md5

Cool. Thanks for giving me the link. Anyway, is this the official GitHub account for GOG or has someone else set it up?

It's unofficial. It appears to be updated weekly though it relies on community effort rather than GOG themselves.

I see. Thanks for clarifying that. I would prefer an official source but this works too. Do you have any idea where they happen to get these hash values? It's not mentioned at all on the GitHub page. Though, I checked some of my games against the list and it seems to match.

I think it's collated from .xml metadata files from the GOG website itself:-
https://openuserjs.org/scripts/xmanacollectorx/GOG_Checksum_Lookup

Edit: By that I mean start downloading an offline installer file, then in your web-browser's download section select "Copy Download Link" then paste that into the browser's URL bar, add .xml to the end of it then press Enter and you should see an xml "webpage" open starting with a line like (example for Deus Ex:-)

<file name="setup_deus_ex_goty_1.112fm(revision_1.6.3.0)_(51757).exe" available="1" notavailablemsg=""
md5="9dd2b837300bfa19c6b5b8fde5d38df6" chunks="53" timestamp="2021-11-26 14:24:56" total_size="550072224">

Do an .md5 checksum of that downloaded file and it should match (as should the "total_size" file size in bytes).

Okay. It seems you can check the integrity of your installers, dlcs, patches and extras with lgogdownloader if you run the command "lgogdownloader --status --no-status-check --game [game_name]". I've tested it and it works pretty well.

Thanks to AB2012, Timboli and direspirefirewire for all your help! :-)

Docs

$ lgogdownloader --help

--status Show status of files

Output format:
statuscode gamename filename filesize
filehash

Status codes:
OK - File is OK
ND - File is not downloaded
MD5 - MD5 mismatch, different version
FS - File size mismatch, incomplete
download

--no-fast-status-check Don't use fast status check.
Makes --status much slower but able to
catch corrupted files by calculating
local file hash for all files.