As already mentioned in the multiplayer topic, there is a LAN mode that you can use without Galaxy: https://www.gog.com/forum/dying_light/multiplayer/post6
However, it is not yet confirmed if LAN mode between GOG and Steam users works.
If you see restricted multiplayer as DRM, you should know that this seems to be very common for GOG, and there are worse examples on how DRM in singleplayer and multiplayer is affecting games on GOG. E.g. Age of Wonders 3, where one had to create an online account for SAVING an offline player profile (this has been changed in an update), and although there is a multiplayer via hot-seat (IMHO obsolete), there was no LAN mode at all, only online multiplayer with need of creating an account at the developer.
Maybe you want to read carefully about a game in the future, before buying it on GOG. They changed their former attitude during the last years significantly (regional pricing, "DRM")