Ways of getting gem:

1. Remove all the obstacle and hope that you get good amount of gems from it.
2. Pray to Supercell RNG to drop gem box to you, and never open it since you need to brag about how many you have.
3. Pay, well it's a pay to skip farming game. What else?
4. Gem hack that uses survey. Well, hopefully it works as I never found a working one.
5. Play Store's AppNana or iOS AppBounty. Download the app and collect points via downloading app and trying them and get points. With enough points, you can buy Google Gift Card or itunes Gift Card and pay for your gem.

My AppNana points is n1484343. Use this code and you'll get 2500 points (so do I). Ask me anything if you don't understand.