I just received an answer from the GOG support about the DLC not working without Galaxy. Apparently, they are aware of the problem and are working on a fix. ;) I guess it'll probably be fixed very soon!
It's a good first step. However, even though this is Saturday, an official statement is yet to be made. The game is still being sold without any warnings at the store page. Yes, the whole covid thing makes things trickier, but GOG is not an indie store anymore, they are quite big and quite capable of addressing users on something so significant as this.
I have already issued a refund request pending an official word from them regarding a fix. If they fix it great, I will cancel the refund request. If more people do this, it will surely grab their attention and (maybe) prevent further incidents like this from happening (this is not the first time and if anything it's getting more frequent). It's also not about doomsday cries and hysteria, as mentioned earlier in this thread, it's about a store selling something they did not deliver (DRM-Free). This was not an "ooops, look at that!" thing, nor is it Squeenix's whole fault, as GOG went to the trouble of wrapping the system calls to bypass some of the game's authentication, so they knew what they were doing.
Regardless of the outcome, what happens after this episode is that every game, from here on, will require community feedback before purchase regarding if it is truly DRM-Free. GOG's word is no longer enough (check their official response here: https://www.gog.com/forum/general/dungeon_siege_and_deus_ex_series_now_available_on_gogcom_f6c95/post152