As much as I'd like to know as well I think that Gilligan will keep his past ambiguous. He mentioned in an article that he sees Gus' past as the briefcase in Pulp Fiction, it's more better if the audience decides themselves what he was before so I'm not having any high expectations for that resolution. Not to mention if they go with Gus' contacts from his past out for vengeance it'd be a threading in the same water and since it's the last season I think it's best if they keep it moving forward with the story of inevitable Walt's downfall.