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.
Also there's a pretty big difference between downloading the installer and backing up the installed files, one is an intended backup solution, the other is a workaround.
Which gog games have DRM? The costumers over there even protested Hitman's inclusion in the store precisely because without internet you can't unlock anything in the game, GOG even removed it from sale.
Problem there is the games I have in Steam which are Secret of Mana, Trials of Mana, and GTA 5 I was looking at and thinking about whether or not to get, are not coming up on GOG.