It's a matter of fact versus fan theory. You said that Gerald canonically studied the Chaos mural. We don't know that. The game never says that. All we know is that Gerald knew enough about Chaos to create the Artificial Chaos series. We don't know that he studied that one specific mural of Chaos from Sonic Adventure. That's conjecture. I'm not saying there's anything wrong with it as a theory, but it's not canon. It's a mystery. There doesn't appear to be any official explanation. Which guide books? Is there a scan or quote available online?