In the flashback when they first pitch Meth to Don Eladio, he tells Gus that he “knows who he really is” but to beware because Gus isn’t in Chile anymore. Then when Gus is being interrogated by Hank they ask Gus why there’s no records of Gus prior to his emigration from Chile to Mexico. Gus’s excuse was that the government was terrieat keeping records, but the comment from Don Eladio implies there was something more to Gus’s past before he became involved with the Cartel. It’s never been explained afaik