Promotion codes are typically given on the official forums/facebook/twitter/instagram of the IGG Lords Mobile. You find your IGG ID by going into settings and then account. It works only once per code per IGG ID. The first box is your IGG ID # and the second is the code listed below. Each site is for different region but works for everyone. Most of the codes should give you around 200 or 500 gems.
Some of them may not work anymore, in which case inform us in the comments below.