Steam is as much de facto a seller of DRM-free games as a electric appliances store is a seller of quake games machines: some people with the right skills might get quake to work in some of the smart fridges or smart TVs they sell, but they're definitelly not made for it, definitelly not sold as supporting that feature and definitelly no support whatsoever is provided for that feature.
When you're making a purchasing decision on their store, Steam doesn't tell you upfront if the game has or not their DRM hence you cannot make an informed decision on that factor: Steam most definitelly do not want potential customers to select games on the basis or absence of DRM.
Also the install process of a game in a new machine with Steam is always via their store which can arbitrarily refuse you access to the games you supposedly bought (only according to Steam, you only "licensed" them) whilst with GOG once you downloaded the offline installer it's de facto yours (even in legal environments where such sales are not treated the same as sales of games in physical media - which are treated as owned). The copying over of a Steam game is a hack, which even without the Steam phone-home DRM might not work, for example, if the game won't run properly when certain registry keys created during install are not present.