It's been a while, but there seem so to be no chance that this will ever get fixed by gog. After poking around a bit, the game seems to call the gog server to auth the user at launch, then it loads your data like unlocked gallery entries, leaderboards and unlocked achievements. If it can't establish the connection, the leaderboards and achievements sections will be unavailable and all the gallery entries will be relocked (tested this with offline installer, w/o galaxy, w/o connection).
I've tried all the available CM builds, cleared the game in online mode launched via galaxy on each single build with the same exact result: Achievement unlocks in-game (no pop-up in the client), gallery entries unlock too although they seem inconsistent (e.g. Tizoc sometimes unlocks sometimes not, despite it's a guaranteed story matchup with Butt). After I quit the game and relaunch it, it goes back to the previous state with all the changes being lost.
I've tried with cloud saves both off and on, no change. Somewhere on the gog server is my garou user database that's either corrupt (some gallery entries are unlocked, no achievements though), or the whole system doesn't work as intended at all. I understand that this way of dealing with player data/progress is quite unorthodox for gog (no local save files it seems, except for config/settings). However it's annoying since on steam it works exactly the same way, with a difference that it actually does work (their server is saving the progress, triggering achievements, etc).
I might try to finish the game with another character or two just to see if any new gallery unlock "sticks", but it won't solve the problem with achievements. I have no idea how to reach anyone from gog tech support who knows the backend and/or is able to check/fix issues on that level.