This is very misleading and simply wrong.Just let me remind you that Germanic languages are all derived from Persian Avestan and also the fact that Zoroaster composed the Gataha in Gothic language proves the fact that Gothic people were originally of Persian /Aryan stock.
German is a child of Proto-Germanic which is in turn a child of Proto-Indo-European. Avestan and German belong to 2 separate branches of Indo-European tree, and they are related only through a common ancestor, which is PIE. Believing that Avestan is an ancestor language of German is very funny.
Also, Zarathustra did not compose his Gathas in Gothic. He composed them in Old Avestan which was also his mother tongue.