Time4Tea: Unlike Steam, once you have downloaded a GOG offline installer, you have the files and they have no physical means to take it away from you. There is no inbuilt mechanism coded into the game that can be used to enforce the GOG EULA.
mrkgnao: The same is true of all DRM-free games on Steam, of which there are many thousands, except that the game you download there comes installed, but if you want you can compress it into a standalone installer by yourself to save space. It's just a matter of convenience; essentially no different.
Yes, that is true for a percentage of the games on Steam. Although Steam makes doing that much less convenient than GOG. It all seemingly requires user-made workarounds and fiddling.
Coming back to my point above regarding licensing: the point about licensing is true for
any piece of proprietary software. Any video game you buy from anywhere comes with a license, as did games that were sold on physical media back in the 80s/90s, console games etc. When we talk about 'ownership' on GOG, what we really mean is that the license is perpetual and not in any way time-limited. It is not dependent on the distributor or developer staying in business, or any of their 'machinery' continuing to function. There is also nothing coded into the game to enable enforcement of the license agreement.
That last part is the essence of what DRM is: something coded into the game that gives the developer control of the user post-purchase and allows them to enforce terms of the license.