The German döner is spit meat in toasted bread with vegetables and sauce. There is no classic vegetarian version of that sandwich, because the spit is the product.
What exists around it is a set of workarounds, and they are not all the same thing.
What a meat shop will actually sell you
A salad box or a fries box with sauce and vegetables, no meat. That is a side that got promoted, not a döner. It can be a meal. It is not the sandwich.
Falafel appears in some Berlin shops as a second protein, usually from a fryer rather than a spit. Halloumi appears in some. Neither is the German döner, and neither is standard. Ask. A shop that only has the spit will not invent a vegetarian cone for you.
Dairy, eggs, gluten
The bread is wheat. The wrap is wheat. Classic yogurt-garlic sauce is dairy. A lot of creamy US house sauces contain egg and dairy. Feta is dairy. Chili sauce is sometimes just chili and is sometimes not.
The meat can be gluten-free if the cone has no wheat breadcrumbs. The sandwich is not gluten-free, because of the bread. Fries can be gluten-free if nothing else goes in that fryer.
A vegan protein is uncommon. A vegan sauce is uncommon. “Vegetarian” at a döner shop usually means vegetables and bread, or vegetables and fries, and you still have to ask about the sauce.
What to ask
- Is there a vegetarian protein, or only a salad box?
- Does the sauce contain egg or dairy?
- Is the fryer shared with meat?
- If they offer falafel: is it fried in a dedicated oil?
If the answer to the first one is a blank look, you are in a meat shop, which is what a German döner shop is.
The honest version
The spit is the product
A shop built around one rotisserie will feed you without meat if you ask. It will not have a second cuisine hiding behind the first.