Since it's missing from the OP, here's the thread on GOG's own forum where people report those issues, and with the Google doc attached to the opening post:

https://www.gog.com/forum/general/games_that_treat_gog_customers_as_second_class_citizens_v2/post1

I guess games being older versions than on Steam? And maybe unavailable features and content?

It's a spreadsheet of games along with omissions and missing features that are only missing on GOG.