Not working as of 2024-05-05. Thx for trying though.

This addon does not appear to work anymore with the current GOG website.

Edit: Updating review. New version posted about two months ago did not fix the issue, but an update since has. To be clear, the correct SteamID was being used at all times so it was purely a matter of addon updates resolving the issue.

That said, I have two concerns with the add-on.

First and foremost, the addon is currently about 80-90% accurate. Game collections, such as the Dungeon Siege Collection, don't show up as being owned on Steam when you own all the components. Likewise because "Demonicon" is "The Dark Eye: Demonicon" on Steam it's not getting flagged as owned on Steam either.

This is somewhat exacerbated when you do a fairly wide search, such as strictly for "RPGs," and GoG's search results spit out 4 different versions of the same game but only one shows up as being owned on Steam. While this is technically accurate it's also somewhat misleading. You might own Pathfinder: Kingmaker and all relevant addons on Steam, but because that's just you owning the base game and DLCs it's not any of the specialty editions, which may not even properly translate from Steam to GoG, so they'll show up as unowned despite owning everything in them.

The second, and lesser, issue is the delay between opening a tab/window and having the Steam/owned icon pop up. The approximately 5 second delay, or roughly 4 seconds after all the page content is done loading. This isn't a major issue that can be somewhat alleviated by opening multiple game pages before looking at individual items, but it can still catch you off guard when a search or the home page loads and you start scrolling through listings before realizing that the icons aren't showing up.

Works great!

Firefox 68.0b6 (64-bit), Windows 10. Получил на steamid.io свой steamID, вбил его в настройки дополнения, сохранил, открыл на GOG игру, присутствующую на Steam - никаких отличий от изначальной страницы не увидел.

never made it to work. no screenshots - so even no idea how it's supposed to.
update: managed to make it work - but it does NOT say clearly in ALL places, that steamID64 should be pasted. also: the interface in itself is TOO basic.. but it works. :)