It's more of a promotional event they have once in a while.

https://www.gogwiki.com/wiki/GOG_Connect has a history of games and dates. If you miss the owning that game on that then, and remember to check the website, you miss it.

I rarely use GoG but every couple months when I log in there's a notification that a game or two from my Steam library was added to GoG.

Oh, it's just for single games. Eh, much less useful then, thanks for the info.