As I said, one strong point is that GOG supports them as DRM-free products, while e.g. Steam (Valve) does not. So if you run into any problems running the Steam version in DRM-free mode (e.g. because the developer has changed something, e.g. enabled CEG in some version, or the DLC they introduced later requires Steam authentication), you are on your own. You can't complain to anyone because you were never promised it would work DRM-free. In GOG you can, the very least demand a refund in case you feel they lied to you with some game for its DRM status.

See the "Steam games you can run without the Steam client" discussion, the last few pages have several examples of people suggesting some games should be removed from the DRM-free list because they can't run it without Steam authentication. So someone earlier reported a Steam game as DRM-free, but for some reason later it doesn't work that way for another Steam user. Maybe the developer changed something for the game, or the original reporter made an error (e.g. in one case someone pointed out that while certain Steam game can be launched without the Steam client, when you progress in the game, it becomes all borked; ie. in reality you must run it with the Steam client authentication).

Also as I asked, how about the other stores that advertise DRM-free games, like GamersGate, Humble Store, itch.io etc., do you similarly feel they should not advertise their games as DRM-free, because they are DRM-free elsewhere too (including GOG.com)? See e.g. the itch.io FAQ at https://itch.io/docs/general/faq : So shouldn't itch.io mention that they have DRM-free games in their catalog, and Humble Store should remove their DRM-free filter in their store?

Similarly, shouldn't itch.io mention (as their selling point) that for many games you get both the DRM-free itch.io installer version of the game, as well as an extra Steam key, because many other stores do that too (e.g. Humble Store)? Itch.io should mention that only if it was their selling point, and no other store does the same, right?

Do you see where I am getting at? Your original point seemed to be that a store should advertise certain service or feature only if they are the only store around offering it, but that doesn't make much sense.