doomicle: No, my original point was that they're already doing that while claiming to their customers that they're not... I don't know why that is so hard for some people here to understand. I'm going blue in the face repeating myself here.
Maybe other users are not as stupid as you seem to think, and know and are fine with what GOG is offering to them. That is, DRM-free singleplayer. As mentioned with e.g. Race the Sun, GOG already points out in the gamecard that you need an online account for the leaderboard or browse for user-made content.
You raise this as some kind of huge issue, even though on the other hand you make it clear that you don't care about DRM. Odd. It seems to be some kind of ideological and political dilemma to you. I would understand you better if you were vehemently against DRM in any games, but obviously you are not as you are happily using Steam too, and suggesting GOG should start selling DRM games too (which according to yourself they are already doing, so...?).
doomicle: But as Jmich pointed out. "DRM Free" is a term that is devoid of a consensus definition. Therefore if that's correct, anything is DRM Free if I say so, anything on Steam, Origin, Half-Life2 and Battlefield 4. It's all DRM Free if someone says it is, because there is apparently no actual definition of what is DRM Free and what isn't.
As far as I've understood, Half-life 2 actually is considered DRM-free nowadays. After you have downloaded the game from Steam for the first time, you can copy the game as-is to another PC, and play it there even without the Steam-client. I haven't tested that myself, but I recall someone mentioning that. Maybe it is already in the "DRM-free Steam games" list.
Careful now though: I'm referring to the single-player part. HL2 multiplayer will most probably not work without a Steam client and an account. So I guess it can't be called DRM-free in your books after all.
Humble Bundle also routinely sells "DRM-free PC games" where only the single-player part is functional, and in order to play online, you have to install and play the Steam version. Or, create an online account for the online features, like Race the Sun. Is Humble Bundle wrong in advertising them as DRM-free games, lying to its customers?