Hank and the DEA managed to find documents to show that Gus was from Chile, spent some years in Mexico, then moved to the US. However it seems that we are never really told what Gus's back story was---or perhaps I simply missed it.
It seems to me that he is obviously not from Chile, because he speaks Castellano/Spanish with a heavy American accent. (But correct me if I'm wrong about what sounds to me like a heavy American accent---I am not an expert in the Spanish language.)
So, what is his actual back story? Where is he actually from? (It is possible that this question is not answerable, but perhaps there were some clues that I missed.)