Erpy: This is a well-known difference between Steam and Gog. Gog likes to personally test each update/patch that developers submit before adding it to the users' library. Steam allows developers to directly upload patches/update their game without enforcing any direct QA process. As a result, patches/updates take slightly longer to appear on Gog and some developers also opt to avoid releasing a torrent of smaller updates that are common on Steam during the initial few days, bundling the updates into a larger patch for Gog after the Steam userbase has confirmed that it's not breaking stuff.
Steam receives faster updates, but they sometimes end up being cast in the guinea pig role. Gog is slower with updates, but the updates they do get tend to be solid.
Thanks for posting this. This actually makes sense & explains why there's such a discrepancy between the dates when patches go live on Steam and when they go live on GOG. Only thing it doesn't explain is the cases where a patch is not on GOG years after it's on Steam.