Gyros is made from spicy, marinated meat. Döner is made from just meat and fat, without any spicing or marinating or whatever. Gyros is made from pork, döner is a mixture of lamb and beef. They’re also served a bit differently.
They’re plenty different originally. In Germany they’ve been bastardized a little bit and brought close to each other. And then Germany went and declared they invented döner kebap, which is of course utter bullshit.