The only difference between Steam and GoG is that your browser is already providing the VM to sandbox (albeit without hardware enforcement) GoG's code running on your machine, while for Steam you have to supply your own VM if you want that degree of isolation. (If you happened to be running Qubes OS then this matter of degrees of convenience would be trivial.) In either case, running some of the store's code on your machine in order to obtain the product is unavoidable. In neither case is it DRM.