Great question — answering as the UphillTech team:
This was left intentionally vague. Says Giancarlo Esposito (who played Gus): Q: What is the nature of Gus’ relationship to his old partner? There was a lot of speculation that perhaps there was a romantic connection in their past. GE: [...] I loved the fact that that came up so strongly, because there was genuine love there between Max Arciniega and Gustavo Fring, and I just adored James Martinez, the actor who played that with such grace and aplomb. It certainly came to my mind, because in my early hopes for Gus, we were at his home... I really get the idea that Gus is married to his work. Again, my theory is that nothing’s black and white , and the love he shows for nurturing another...
Full answer by Walt: https://movies.stackexchange.com/a/47024 (CC BY-SA 4.0).
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