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.
In progress: Aoba Suzukaze (New Game) | CATS (Zero Wing)
Possible plans: Chiaki Nanami (Dangan Ronpa) | Game Gear (Sega Hard Girls) | Namco (Consolers) | Undyne (Undertale)
My webcomic, Consolers. It's about game companies