What are Lua's string.gsub pattern rules?

Question

Here is the current script that have running:

var_name="[Clan] Imposter"

     while var_name:find("[Clan]")~=nil do 
          var_name=var_name:gsub("[Clan]", "") 
     end

print(var_name)

I was expecting var_name to be "Imposter" however the result was "[] Imposter".

How do I get the result from this function to be "Imposter"?