Alright, well two things. First, I would just put the file (and rename it) in lua/autorun/force_download_workshop.lua
Secondly, this most likely will not make a difference, but you always want to make sure. At the top of the file, do
if CLIENT return end
Thirdly, make sure that the addons you're wanting to add are as well on your server, or your server's collection.
Lastly, some of your addons, such as this one:
http://steamcommunity.com/sharedfiles/filedetails/?id=433013068 require more addons, and in the description of this addon, he says you need more materials ect, but the ID on his description didn't appear to be in this list of addons, so that's something else you're maybe forgetting.
P.S. Claptrap sucks.
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The way you described that though, it makes it sound like your client is having issues downloading that all, maybe it's just too much for Garry's Mod to handle, as to be honest, garry's mod barely work as it is. [/code]