That’s not true. You still only receive a license to play the game, you do not own it. Directly from GOG’s website:
We give you and other GOG users the personal right (known legally as a ‘license’) to use GOG services and to download, access and/or stream (depending on the content) and use GOG content. This license is for your personal use. We can stop or suspend this license in some situations, which are explained later on.
Practically this means you cannot resell your GOG installer in the way you could resell a physical book.
Absolutely. GOG has a much better license and distribution model, but it’s still a license.