What kind of soda is gluten free?

Probably all sodas. Probably all sodas available at stores are gluten free. Sodas are just sugars, carbonated water, caffeine, etc., but rarely would have wheat, rye, or barley. If someone was selling soda with real malted wheat or barley as flavoring, that soda may have gluten. Corn is gluten-free.
Gluten. The most popular sodas — including coca-cola and all carbonated sodas from pepsi co. — are considered gluten-free to 20 parts per million.