Pliky: While optional for single player games/game modes, Galaxy is mandatory for multiplayer in some games. An... interesting choice for a so-called DRM-free store.
Earlier GOG solved that problem by simply disabling the whole multiplayer part in games, I recall e.g. in Full Spectrum Warrior and many others.
Many people objected to that as they saw the GOG version incomplete due to lacking the multiplayer part.
I was (and am) fine with both approaches, either keeping online multiplayer support (which requires an online client to run), or disable the multiplayer part altogether. Easy for me to say because I don't generally care for the online multiplayer part in (GOG) games, I buy them for their single-player experience.
As long as that single-player part is DRM-free, ie. does not require me to log into some online account in order to play it.
As for Linux support, I am practical and understand GOG probably doesn't have money and resources to concentrate on Linux support, due to the little extra income it could make. I still think it is Linux' responsibility at this point to be able to run Windows games (WINE etc.), rather than developers and stores making Linux ports of their game and actively supporting Linux. That is the feasible way to increase the number of Linux players.
Then, when the critical mass is reached with the Linux gamers, the developers (and stores) would follow and start making more and more Linux ports and offering Linux support.