If you are running a dedicated server and are using fast download (which is where you redirect your clients to an outside host to download client side files such as maps and stuff) You also have to put your clientside lua cache on that fast download server. This would be the garrysmod/cache folder. Inside that folder is another folder called dua/ and in that folder are all of your cached clientside lua files...
Sorry if this doesn't make sense... long story short... if you use a fast download folder on an external server.. you have to ALSO include the garrysmod/cache folder on that external server to make any scripts on your server that have clientside portions work correctly on the clients that connect.