The last time this game was on sale on GOG was 30 March 2020 -- just over 21 months ago. Since then, it has been on sale on Steam 15 times, with the most recent sale price being $5.99 (80% discount).
Contrary to mamasume111's opinion, I'm sure many (probably most) people would agree that charging GOG customers five times as much as Steam customers for the same content, with the only difference being DRM-free on GOG, is unacceptable. I don't like it, but I can accept paying more to get a game DRM-free. Five times as much, however, goes far beyond any reasonable premium, especially when the non-sale price is the same on both stores.
It is quite obvious that the publisher sees no value in GOG or GOG customers.