Can you link to the claim that GOG is the only DRM-free digital distributor of PC games? As such, it is not a correct claim, but:
kbnrylaec: An GOG staff claimed that GOG was the only DRM-free digital distributor for PC games.
No, it is not.
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Steam No, I am not joking. Steam do have many DRM-free games.
Once you downloaded those true DRM-free games, you can backup them as you like, and run the game everywhere without Steam client installed. [1]
I think it is important to note whether the service/store sells and supports the games as DRM-free products. Steam does not. Steam service is merely not forcing DRM over all their games but lets the publishers decide, but they offer tools for it (CEG) and also allow the use of third-party DRM, and if you have problems running some game "DRM-free", you can't expect any support from Valve (nor the publisher) because you were never promised they would work as such.
For that reason, I don't count Steam as a"DRM-free digital distributor", even if it has some games that can be used in such a manner. If you list Steam only as "well, they have some games where the DRM component is not used", then I guess you should list also e.g. EA Origin, as they have some DRM-free games as well.
But besides that, yes there are other stores too which offer DRM-free games. DotEmu also used to be one, but they closed their doors, as did Desura. Gamersgate even lists (or listed) some of its games as DRM-free, but it was a bit odd thing, you needed certain workarounds to make them so (ie. copy the game installation files from the temporary directory somewhere else before you exit the download client, or some such shit).