The game took the gems I paid for. How do I get a refund?

If you did not get the items you paid for in a purchase then you will need to inform the game support. You should have a mail from them confirming the purchase in the game messages with the purchase details.

Check the support page for the best way to contact them.

If they do not help then check in your app store (as they will be the ones that processed the payment) to see if you can request a refund through them. But you should try to claim the gems from the developers first as making a refund for a purchase that they believe did go through may result in a ban.