(Spoilers):
I find this to be one of the most intriguing lines in the series. I have formulated a couple of theories as to who Gus is, or more exactly, what he did before emigrating to Mexico in 1989. I’m not sure any of these theories work.
I want to see the rest of season 5 right now, because it seems like this is going to be explored a lot more in the coming episodes.