WinterSnowfall: "Owning" something in legal terminology would mean you are the IP owner, which is of course nonsense. You haven't bought the rights for a game and plan to make a sequel, you simply own a copy, and are subject to an EULA (or you know, not really :P).
De facto, since your GOG copy is offline, you can escape with your "booty" even if the EULA changes, the IP changes owners, world governments return to a feudal lord system etc.
Tax official of the royal GOGBear: Pay thine tithe for yer' digital merriment, peasant! :P
GOGlodite: Nay, m'lord. Thou hath sold me the game in the fore years, the golden time of the internet, and I have faithfully kept my backups.
Technically, ownership laws have well understood, intuition friendly, implicit properties that a court will uphold in the absence of any explicit agreement. They dispense of the need for complicated legal constructs like EULAs which is a big part of the charm.
Now, that mostly existed in the context of physical goods and the digital realms introduce some complexities (for things like borrowing for example), but honestly, I think there should be a way to translate ownership laws from the physical to the digital realm with most of the properties kept intact.
Otherwise, for licenses (which are still useful to allow some use cases not well covered by ownership law like copying open-source code for example), there should be a limited number of well-understood ones recognized under law.
You shouldn't be able to force on your users the cognitive overhead of a customized EULA.