With GOG, you can buy any game, and you’ll have files to keep. Once you have the installer, you can keep that forever.
Even if your GOG account is hacked, banned, and GOG goes out of business, you can forever install your game onto any compatible machine, even offline, and play the game.
That’s what GOG does differently.
It’s like buying a physical game, except there’s no disc. They can’t revoke your access or deactivate your ability to play the game.
That’s just semantics.
When you buy a CD, you don’t own the songs.
But you do have some item that belongs to you.
With Steam, you have a ticket that will let you into Steam to download the game for as long as your account is in good standing and as long as Steam exists.
With GOG, you have a file you can use to install the game on any machine INDEFINITELY. GOG can’t revoke your access for any reason, and if GOG shuts down, you can still install the games.