you have to just sign up using your google account after sign up there will be three ways to earn free gems for clash of clans

first is by watching rewarded video ads

second is by completing easy and fun surveys

third is by playing amazing and fun games (under development)

after completing any of the task listed above, you will get rewarded by gems

and after you have enough gems that is 150 or 300 gems, you can click on redeem button then we will send you a google play gift card to your email of that amount to purchase that much gems.