In classroom interactions that take place over video conferencing platforms, teachers and students continue to gesture, but their bodies are neither physically copresent nor fully visible to each other. Do instructor gestures help learning in this context, as has been found for in-person learning and for video-based learning in lab experiments? We showed professors lecturing spontaneously with unscripted co-speech gestures. In some conditions, we cropped the video so only the top half of the professor's gesture space is available, or removed the video altogether. Results from our between-subjects experiment show that participants paid significantly more visual attention to the partial gesture condition than to stimuli where the gesturing was fully visible, and they scored significantly higher on an immediate comprehension test if they had seen lectures in the partially visible condition. This work raises further questions of how gestures help learning.