The Google Play Store tries to improve customer security by applying a function that keeps an eye out for unusual shopping behaviour. By trying to purchase something out of the ordinary, you may prompt Play Store to give you a "High Risk" warning and cancel the order.
Rest assured that this has nothing to do with the security of either Evoker or the Play Store itself. Simply put, it is just an additional layer of security that prevents unauthorised or unintentional purchases. Please note that it is possible that the Google Play Store still puts a "reservation" on your credit card, equal to the value of the purchase. However, this reservation is temporary only and will be cleared automatically.
Unfortunately, you will not be able to buy Gems again immediately. In theory, the pop-up will ask you to wait for 30 minutes, but this might actually take a little longer. Additionally, you might double-check and see if your billing address and your name registered in your Google Wallet match the ones with which you provided your credit card company.
If you still experience problems with the "high risk" pop-up when trying to buy Gems, we advise you to get in touch with Google via https://support.google.com/googleplay/digital-content/?hl=en for further help.