Soon, GOG and all other storefronts will state that you're purchasing a temporary digital license for any game who's publisher uses an EULA that states you don't own the game. This is due to the recently signed California law that forces storefronts to be transparent about the publishers EULA.
But also with GOG you can download the installers and play offline. It's literally one of their big selling points. It's less convenient than things like steam, but you can do whatever the hell you want when you buy it. So in that regard, it literally is a purchase. Or as close as you can get with digital goods.
On a legal level, it is how GOG works. They still only sell licenses. You just have the loophole that their installers and the games installed by them will work regardless.
Legally, it's still a license, it's just effectively impossible to revoke.
Edit to expand on this: A truly offline forever-purchase of physical goods can be re-sold. See https://en.wikipedia.org/wiki/First-sale_doctrine (this is the US-specific version, other jurisdictions may have similar doctrines).
American legal concept that limits the rights of an intellectual property owner to control resale of products embodying its intellectual property.
A digital "purchase" is usually non-transferable, even from GOG. It can't be removed from your own HDD once you download the installer, but there are still restrictions attached on what you can do with it, even if those are limited and hard to enforce.
Technically, probably yes, but you can buy old, opened games on eBay. I doubt you can do the same with GOG games. Digital media is much harder if not impossible to resell.
A small minority of GOG games have DRM, a majority of Steam games have a form of DRM. "Use a simulator" isn't a solution, I shouldn't need a third party program to play the games I paid for.