If a child is naughty, you might spank it on a soft spot. As the gorilla is lying on his back, his soft spot is hidden. You need to expose it. Perhaps you could try rolling him over.
I tried "roll gorilla", "roll gorilla over", even tried using the bamboo stick as a lever :P.
The words you need are in the instructions, so I would push you towards those and then you can pull out the verbs you need.
Fantastic, I finished it! Wonderful stuff! Thanks so much for the hints :).