I'm watching the 1974 version of Gatsby, with Robert Redford, Maia Farrow, Bruce Dern, Karen Black, and Sam Waterston. I forgot that one of the rumors about Gatsby is that he's German, and that's how he got all his money; he's related to the Kaiser.
The woman who tells this to Nick Carraway, the narrator, whispers it, as if it's scandalous, which it would be in the 1920s, so soon after the Great War.
We also know that Nick was in the war, so his reaction - or lack of one - is actually interesting. He knows it's just a rumor, but if it isn't, he doesn't really seem to care. Germans to him aren't really an enemy because, perhaps, he has dealt with real Germans, not just stereotyped versions as portrayed by the yellow press.