Can I get some gems without using real money?

You can get gems by completing achievements. To find your achievements you go on single player or multiplayer and click you player icon. Then you go to the section where you find the chests and look to the left and your see a button with a trophy on it. Click that and the achievements with the stars on them are your completed ones. Click on those ones and you will see that you can claim diamonds as a reward for completing it.