I think this looks really promising! I think your idea is fleshed out, your story is compelling, that concept art looks fantastic (in particular the "nonbinary??" note gave me a massive smile), I think you're doing great! One thing I didn't spot directly in the GDD would be to have content warnings displayed on the title screen or splash screen, and I'd highly encourage you to include those (potentially even in the prototype), as you're telling a story dealing with issues of abuse and suicide, which is a sensitive thing for some folks. Other than that though, I'm excited to see where this goes!