So, you want to go as an original character? Yourself as a Gryffindor?
I'd say get a Hogwards school uniform costume, and a Gryffindor scarf or tie or something.
Apollo Justice (Ace Attorney)
Possible future cosplays:
Terezi Pyrope (Homestuck) - Derana(original character) - Vanellope von Schweetz (Wreck-it Ralph)