Also, sometimes games on gog don't receive DLC or patch updates because the devs/publisher forget or don't care. I remember seeing an extensive list of games, ~250, explaining what they had missing. I think A Hat in Time was one of the worst offenders
Here's a gog forum post on the problem - https://www.gog.com/forum/general/gog_is_it_truly_more_difficult_for_developers_to_update_their_games_on_gog/page1