If a player has the css game mounted, if they have it purchased and downloaded, they can see the models and various assets. Otherwise, they will see errors and missing textures. Try not to pakrat Valve’s content. I don’t think they take kindly to that.
So on these gmod servers where there are cstrike assets used the server operators just assume that everyone playing has cstrike? If someone were to join that particular server without CSS then they’d just get all errors for the missing assets? Seems a bit odd to me that a server would do that but the concept makes sense.