StingingVelvet: You absolutely need to sign into Galaxy to get them, but I am saying once they are downloaded I am sure they work without it. If they do not, if you need to verify them every time you play through Galaxy, then I would have a problem with that, absolutely.
TerriblePurpose: I replied in another thread, but I'll reiterate here: it looks like, from what GOG has described, that this is absolutely DRMed. If you take your offline installers and install onto an offline machine, you won't be able to access that content, because it requires an on-line authentication via Galaxy. That's pretty much the base definition of DRM, IMO.
I think what StingingVelvet is saying is this: Once you use Galaxy to download and authenticate the Rewards DLC the first time, you should theoretically be able to move those downloaded files (as a ZIP, EXE, or individual files) to an offline installed version without ever needed to re-authenticate.
Here's an example: Jim has two computers. Computer 1 (C1) has Galaxy on it, whereas Computer 2 (C2) doesn't and only uses the offline installers moved to it from C1.
If Jim uses Galaxy to download and authenticate the Rewards DLC on C1, he should be able to then move and use that DLC on C2, where the game was installed using the offline installer.
Like StingingVelvet said, this assumes you only ever need to authenticate the DLC once. Even then, you could consider this DRM, but at least it's a one-time form of DRM and can be used elsewhere once it's initially authenticated. Admittedly, that doesn't really help if you have a zero tolerance stance on DRM, though.
If, however, the Rewards DLC requires an always-online connection to continually (or periodically) authenticate, then that's a real problem considering GOG's stated objective.