Most of the time the level you are trying to download, requires custom models, sounds, or textures, only available when you download the map from garrysmod.org. That has always solved my issue.
Not many SnowPL maps yet. Or atleast any good ones. What's stopping you? I'd make one if I 1) had the time 2) was creative and knew how to make a good map. I can make a map within a week. no textures No gameplay One Stage. Have you ever made a map? And what makes you think I want/have time to make this? I'm already working on a PL map, on my speed.
- Meet the Spy - L4D1 trailer They are all rendered ingame, sure, they made a few high quality skins, effects, and animation, but it's rendered ingame.