Leroux: You're all trying your best to talk past each other. :P
When you're talking about "ownership", amok is concerned with legal issues, while you and others are concerned with more practical matters.
toxicTom: Not really. Steam can revoke a license. "Enter physical game store name here" can't. GOG can't. Buy to own, it's on their front page - and it's legally binding. I believe even if they suspended your account - by that phrase on the front they would need to give you a last chance to redeem your "owned" games when they shut you out. So I'm talking about legal issues.
amok: you are wrong, actually. Buying a licences on Steam gives you the (permanent) access to the license and to use it within the bounds of the license agreement. However, you can loose access to the license via Steam, in the same way you can loose access to a license via gOg (gOg can even remove a game from you library). The difference here, again, is not in the ownership of the license, but in he enforcement of the license agreement.
toxicTom: see above.
You are right that gOg can not revoke a license, however you are wrong in that Steam can do so - they can not. Only the rights owner can revoke a license.
The difference is in accessing the game you have the license for through their service. Both gOg and Steam can suspend that right to access, i.e. you can no longer use that service to access your game license. There is still no difference between gOg and Steam so far, and we now no longer talking about the ownership of the license, but the access to it. The difference is in the DRM, while on gOg arguably their service ends when you have downloaded the game, for many cases on Steam it do not. Games there still draw on the Steam service after they are downloaded (like it or not, but that's a different discussion...) and revoking the access means deleting it from the Steam library. The DRM free games on Steam do not need to do so (you can copy them out of Steam) and they effectively do not draw on Steams services to run, so they behave then same as the gOg games (when you move them out of the Steam folder i.e. no longer make use of Steams service. So effectively, there is no difference in the ownership of the license between those two services.
amok: yeah... I am sorry for splitting hairs, but I really do dislike calling buying a game on Steam for 'renting'. It erks me. For me it is just propaganda, presenting something as something it is not to twist an argument. While propaganda in itself can be seen as neutral, I do dislike its use.
Call it enforced license, call it lack of control, call it... heck...DRM if you want. All those things are correct. Renting it is not.
(but I may argue against the DRM label on Steam just because Steam :))
Starmaker: It's grassroots propaganda, but "buy a game" is propaganda too. Copyright holders say "buy a game" and "you wouldn't download a car", but are the first to point out games don't behave like cars at all when they are told to pay up. I suggest "buy a license" (or, as it may be, "licence").
Indeed, both gOg and Steam are technically wrong when they say "buy a game", and I should not use the term also. I will keep an eye on it, so it do not happen again.
amok: gOg can also cut me out of my library (it is in the ToS), or fold, and if that happens and if my house burn down - I can kiss my DRM free library goodbye also.
HereForTheBeer: Yes, your house can burn down after you get booted. Not gOg's fault if one didn't make backups, and their system would allow you to continue to enjoy those games long after said booting and subsequent fire / other disaster so long as you had those backups. And even then, you might be able to appeal to gOg to get the installers for which you paid money.
So yes, they can cut you out of your library, as in you no longer have access to the files via the website. If you already have the installer then you can continue to install and play the games just as before, long after you have been booted and long after you no longer agree to the site's ToS.
And yes, gOg could fold. And if it does, you can still install and play the games so long as you have the installer.
The difference is in enforcement of the post-purchase EULAs. gOg does not have a mechanism to do so (beyond not installing that particular game if you don't agree at the time of installation), and Steam does - which it can enforce, incidentally, long AFTER the game(s) is / are installed - via the client. Doesn't matter if you agree with the game(s) EULA; the client itself can lock you out of your previous purchases.
Tell you what - try this simple experiment: the next time you're greeted with a new EULA upon login to Steam, simply choose to NOT click the Agree button. And then try to install and play one of the Steam-client-required games from your library. Next, logout of the gOg website and turn off the downloader, install any game for which you have downloaded the installer, and then try to play it.
You'll see the difference: one has you playing the game and the other has you cussing-out a certain industry titan.
Indeed, and as you yourself say, the difference between gOg and Steam is not in the ownership of the license, but in the enforcement of it :)