Now, now Achim and Alfred...
Goertz was a naturalized U.S. Citizen, not a birthright citizen (like me and others born in the USA) and that's the distinction. I don't think anyone, Achim, would NOT call him an American after he obtained his citizenship, a right and privilege he accelerated by serving in the US Army. Being born in Germany, one could easily say he was German; later being a US Citizen, one would also call him an American. Thus, perhaps we should call him a true German-American, being born in Germany and a German citizen until that was relinquished upon his naturalized U.S. citizenship. Getting into the nuances here, a birthright citizen can't have their citizenship revoked; a naturalized citizen can under certain circumstances mostly having to do with either misrepresentation (intentional or otherwise) on the application or dishonorable discharge in the armed services.
One should note, though you have to read between the lines, that he immigrated to the USA in 1936, AND his mother's family was of Jewish heritage. Had he stayed in Germany, who knows what might have happened to him; throughout the 1930s things started to get very dicey for those of such heritage. Thankfully he did not stay, and we are all better off for the legacy he left behind with some of the most beautiful and iconic car designs. His BMW 507 design is as lovely as the Pagoda--different, but beautiful. Right car at the wrong time.