Just speculating here, and not a strong Swedish speaker, so please do correct me if I'm off on this. I'm guessing that the pronunciation and spelling of English and Swedish plays a role in understanding how someone might get the name Justis out of Gustaf.
In Swedish, the G in a name like Gösta or Göteborg is not a hard G sound but more like a "yuh" Y-sound. I'd guess the name Justis or Justus would also be pronounced with a similar Y-sound, or maybe closer to "yoo". But in any case, the name wouldn't be a homophone with the English word "justice" but sound more like the name "Eustace". I'd guess the name Gustaf could have been pronounced sometimes with a softer-G as well. Hearing it this way, or feeling the sounds in your mouth, it is a more natural step to go from Gustafsson or Göstasson to Justisson. And from there the step to Justis is just as natural.