Yes, he is considered Danish despite writing in German in later life
No, he's considered a German writer
No, he's considered a Scandinavian writer
No, nationality does not affect writing style
Answer
The answer to this question is yes, Gjellerup is considered to be a Danish writer. Despite the fact that he wrote in German in later life, he was born and raised in Denmark, and his works are often considered to be part of the Danish literary tradition.
The Enigmatic Tales of Karl Adolph Gjellerup: A Journey through Danish Literature