Gandalf is a famous mage. He may know Chris through various channels. However, the exact reason may vary from job to job. In some works, Gandalf may explore unknown information by communicating with other wizards, studying ancient documents or using his magical power. In other works, Gandalf may have direct contact with Chris. For example, in some novels or games, Gandalf may interact with Chris in the story to understand his background and identity. It should be noted that Gandalf knows that Chris's reasons may vary from work to work, so the specific details may vary from work to work. If you are interested in a specific job or background, I suggest you consult relevant materials or experts to get more accurate information.