Instructor behaviour is known to affect learning performance, but it is unclear which specific instructor behaviours can optimize learning. We used eye-tracking technology and questionnaires to test whether the instructor's gaze guidance affected learners' visual attention, social presence, and learning performance, using four video lectures: declarative knowledge with and without the instructor's gaze guidance and procedural knowledge with and without the instructor's gaze guidance. The results showed that the instructor's gaze guidance not only guided learners to allocate more visual attention to corresponding learning content but also increased learners' sense of social presence and learning. Furthermore, the link between the instructor's gaze guidance and better learning was especially strong for participants with a high sense of social connection with the instructor when they learned procedural knowledge. The findings lead to a strong recommendation for educational practitioners: Instructors should provide gaze guidance in video lectures for better learning performance.