In this video (at 3:00) the instructor demonstrates the word klug by pointing to his forehead and making a circular motion with this finger. I've checked other videos on German hand gestures but this one doesn't appear, so is this particular gesture regional or peculiar to the speaker?
Note that English speakers have a very similar circular motion gesture which means "crazy". Apparently German speakers use a different gesture for this where you tap on your forehead, something which an English speaker might use to say "good thinking". Overall, though English and German have many gestures in common, there are are several used by German speakers which either have a different meaning to or are unused by English speakers.