Simple question about 'Error'

Alright so question, when I join a server and Gmod starts downloading all those custom items, I still end up with a bunch of ‘Error’ symbols everywhere. But I was wondering, is it because my Gmod downloaded the items wrong or am I simply missing some major addon that would reveal all those Errors.

It is either an addon, or a game.

The “Errors” are caused by the server having models which you do not have installed onto your garrysmod. They are also caused by you not owning a game which the map or server is using.
You can ask the server player what the models were then search for them on garrysmod.org (www.garrysmod.org)

The majority of players have PHX and Wiremod which use SVNs. (Explained here http://wiki.garrysmod.com/?title=Svn)

Basically it means the server has a model (From an addon or game) which you do not have installed.

Helpful links:

http://www.wiremod.com/forum/technical-support/4-wiremod-svn-guide.html
http://bunglist.com/gmodwiki/PHX3_SVN