Very good. I asked this question several times over the years to people that loudly proclaimed it was a standard feature everyone had. None of those people has answered me.
Took me a while to figure out how GOG does this. If I had not known it should be there, I probably would not have known they can do it. Similar to how I only know about Steam having this feature only because of discussions here.
My guess is, that it has to do with the mechanics under the hood. Steam and GOG have very few tags. It's maybe 200, give or take. Managing negated tags might be easy or cheap or both. Plus I guess those tags are a lot more accurate, because the are curated somehow, so it is also a lot more usefull.
Compare to Itch with thousands of tags and no curation at all, and a catalogue much, much bigger. GOG has 12000 games. That's less than 1% of Itch's catalogue size. Steam has 240000 games.
When I last searched, I could not find that feature in game stores other than Steam. In a store like the Google Play Store you cannot even combine more than one tag, let alone exclude tags. Many other stores did not have tags as seen on Steam or Itch at all. And from what I could gather, the exclude option on GoG is a newish feature. I see comments from people wishing for this feature even in 2025, so maybe it is new or people did not recognise the possibility. (You need to click the open eye near a tag.)