Gumba means thumb in Swahili and that's the idea behind the game, you need your thumbs to play it and a great memory to win it. Each level starts by showing a drawing and you have a few seconds to memorize it, after that you'll need to replicate that using your thumbs (preferably but not exclusively, as in Twister you can use whatever comes handy). The trick is not to lift your thumbs after you started drawing.
Here are some of Gumba's features and what will get you hooked on it:
Play "Gumba" once, play it forever. Your thumbs will hate you but your mind will thank you, a demo proving that is on YouTube. The game is absolutely free (no ads, no IAPs) on Google Play and it’s just waiting for its next victim.