That's the difference between license and owning.

No, when you own a game, you can make copies and sell them. That is because owning the game means you own the copyright to the game.

If you are not the owner of the IP (which you aren’t, unless you own the company that made the game), then the only way to legally play the game is for the actual owner to provide you with some kind of license. If you don’t have a license then the default copyright rules apply which means you aren’t legally allowed to have or play a copy.

Your license is also limited and doesn’t allow you to ‘do whatever you want’. Try selling copies and see how quickly you get sued. You can’t even do what you want with your single copy. Go buy a bunch of physical games and start a game rental business. Or buy a bunch of physical games and open a game cafe where people can play ‘your’ games. Your license doesn’t allow you to do that.

online servers for authentication

I am not aware of any game on GOG that requires an online server for authentication. I'm not saying no such thing exists - I don't own every single game on GOG, but that would go against the whole DRM free thing. Care to name a few games that do this? I don't mean games that have an online mode that require a server, but games that just require authentication against an online server to be able to play the game.

Depending on the era of the game, you might well own a copy of a game on a disk, just like you own a copy of a book when you buy a book. Weaselling out of first-sale-doctrine stuff came a long time after people started buying video games. A century ago, publishers were trying exactly the same thing with books, and depending on the country, either legislation was introduced that made it explicitly illegal, or the courts determined that putting a licence agreement in a book just meant that the customer got a copy of a licence agreement with their book, not that they were bound by its terms.

They don’t have DRM. That’s not the same as owning the game.

That's why I mentioned that you purchase a license. That has also always been true even if you "bought" a game as a physical copy in a store. A DRM-free game is still the closest thing you get to owning a game.

If you don’t back up the games or installers yourself, and GOG goes under, you lose access to your library the same as Epic or Steam going away.

I have heard this argument before, but I really don't get it. Of course you could lose your files if you don't download them. I'd say that's so obvious it isn't even worth mentioning. If you lose or destroy your physical copy of a game you also lose access to it. Pretty obvious.