I start gesture drawing with my 4th and 5th graders in the fall each year, and we go back to them often, as they are quick and don't require any prep and only a few materials. The kids LOVE doing them! To begin, we read and discuss the handout (available in my TPT store ) and I do a quick demo. A student takes an action pose, freezes, and we do a quick stick figure showing line direction of the figure, followed by a sketchy, loopy line that suggests form and movement. The whole thing only takes 30 seconds, and by the 3rd sketch, everyone has got it and they are loving it. We rotate posers and I count down the pose.
Observational drawing packet from my Teachers Pay Teachers shop - includes vocab for both gesture and contour and printable, projectable handouts https://www.teacherspayteachers.com/Product/Observational-Drawing-Gesture-and-Contour-exercises-2656125
Another option is to use wooden mannequins or even sports figures from magazines. Again, start with a fast stick figure to show the line directions, then "flesh it out" with the loopy line, like you are wrapping a string around and around the figure.
In the past, we have turned these figures into Keith Haring style figures by adding a contour outline around the outside of the gesture and filling in with paint. We have also made 3-D versions using a sheet of aluminum foil.