You will occasionally receive Gems as a reward for a level up or for finishing a boss fight.
I addition, you can connect your game to Facebook, which will reward you with 100 free Gems.
The best way to keep earning Gems in-game is by completing card collections in the game. There is a huge variety of different collections to complete, and you just have to collect all the cards in one collection to earn your reward. Individual collections can provide up to 5.000 Gems, and the total worth of all collections in Evoker is over 40.000 Gems. This should keep you busy and supplied with Gems for a long time!
Please note: there are no special deals or offers with outside websites.
Those pages will try to scam you and steal your information and/or your accounts. Never trust any outside page that offers special offers, free Gems, or even hacks for Evoker!