You can get free gems by just downloading Apps on from the Google Play Store or the Apple App Store with FeaturePoints. It is free and you only have to download Apps without registration – you don’t have to rate them, buy anything or even keep it on your device after you got the Credits for downloading it. With the earned credits you can buy giftcards (or use the PayPal credit to buy a gift card) which you can turn into credit for Google Play or the AppStore to buy gems in-game. You can earn up to 100 gems every day by just downloading some Apps.
From all the services like AppBounty or AppNana I like FeaturePoints most.
Just go to FeaturePoints and follow the instructions: