Since GOG doesn't have a built-in Workshop like Steam, you can use the WorkshopDL tool to install mods manually:
: For those interested in creating content, the Serious Editor 4 is available. Beginners can start with basic tutorials on creating levels and using the new "Scenery Mode" for building large environments. serious-sam-4-installation-tutorial
: Buy the game on your preferred storefront (e.g., Steam or GOG ). Once purchased, it will appear in your "Library." Since GOG doesn't have a built-in Workshop like