Gumball refers to Marshall by an affectionate nickname in German, right?
Was Gary German?
I think so? ( ╹▽╹ )
In Fionna and Cake Gary calls his pastry little guys "pastrymeschen", and "meschen" basically means humans/people in German, so I guess yeah, he could be German. People already headcanon (though I think it's somewhat canon) that PB is German&Korean and apply it to Gumball/Gary as well, me included, and that's where this general consensus come from.