There are some ways to get gems.
1. Complete achievements : If you complete achievements then you will get some gems;
2. Remove obstacles : You can also get some gems if you remove trees, rocks and bushes;
3. Gems box : You can also get 25 gems from gems box.
4. Free my apps : Okay this method is very easy. Just go to google play and download "Free my apps".
You just have download some apps to get points. Download apps and run it for 30 second.
You can buy "Google Play Gift Card" with your points.
You need minimum 3000 points to buy a gift card.
You can buy gems with "Google Play Gift Card".