Yeah, and GOG was always saying Galaxy would never be mandatory, at least for offline stuff, GWENT and such would. of course. But it's mandatory for an offline game, now (their own game, even), if you want the complete and full game with bonus. <sigh>
Can't you just log in to your GOG account in the game menu? I'm not sure how it works with the GOG version, but the PS5 version just asked me to log in. I'm sure the Steam version also doesn't require you to run the game through Galaxy, because that'd be a violation of Steam rules (games with third-party DRM must have it stated on the store page)
No. If you look on that page for how to get your rewards, you'll see that if you own it on:
GOG (Store page and rewards page): "Launch The Witcher 3: Wild Hunt via GOG GALAXY 2.0 and start playing. Your rewards will be waiting for you in the Royal Palace in Vizima."
Steam/Epic: "Launch your chosen version of The Witcher 3: Wild Hunt via REDlauncher and start playing. Your rewards will be waiting for you in the Royal Palace in Vizima."
XBox/PS5: "Return to The Witcher 3: Wild Hunt and start playing. Your rewards will be waiting for you in the Royal Palace in Vizima."
Out of those, I expect you to be online to play on Steam/Epic, anyway, and assume that's the case for consoles, too, so doesn't matter much for those. GOG, however, demands Galaxy/online for an offline game that says Rewards are for *owners*, but then goes on to say you have to use Galaxy to get them and if you ever start a new game, you have to use Galaxy again.
The title "Rewards for owning The Witcher 3: Wild Hunt" is misleading, at best, false advertising, at worst, since not ALL owners get the rewards.