You do own the game, for most games sold on GoG. (Not the copyright assignment. The “game”, or “copy of the game”, however you want to refer to it.) If you buy a game as a disc, in a store, you own it until you take it home and find an EULA contract that takes away that ownership. I don’t believe a clickwrap contract has any legitimate effect, but I don’t think the courts would be on my side.