There are a couple of ways you can get free gems: 

1. by collecting your daily reward, 
2. by levelling up the love meters of your matches, 
3. by playing Game of Love, and 
4. by tapping on the Shop icon in the lower-left corner of your screen, and after you do so, by scrolling to the bottom of the page you should be offered to tap on 'Earn Gems.' From there, you can watch ads or complete some offers (if they're available) in order to get Gems.