windowscentral.com/gaming/pc-gaming/gog-game-issues-missing-updates-list
docs.google.com/spreadsheets/d/14haGnU2kcY6wV6ssBeyW6_mMk6kkVqZX-QvMKlgXGu0/edit?gid=0#gid=0
It's up to a game's devs what they provide to GOG, and sometimes they don't provide the full game that you'd get buying from Steam.
I was aware that some GOG games weren't the same as the versions found in other stores, but I never knew the problem was this widespread.This effort to track issues with GOG versions of games actually got started in 2018 in a GOG forum thread that's still alive today, now reaching 400+ pages with thousands of replies.
The spreadsheet catalog contains a list of 1,140 games with at least one issue. There are 1,531 issues in total. Those numbers are not typos. Sure, some of the issues are less severe than others, but that doesn't mean GOG customers should receive an inferior product.