Agreed. A veteran detective who's worked closely with the Batman for at least 10 years is going to know in a heartbeat when Batman says word one. Not only is Dick not as tall as Bruce, but he's much more the gymnast than the power weightlifter. Gordon is going along with it for appearances in front of the department. He's going to wait and see how it goes, because Arkham is still full to overflowing and they need the assist.