For various reasons, some devices are incompatible with Gems of War. As a result, we sometimes remove the ability to install it to these devices via Google Play. This means that if you're searching Google Play from an incompatible device, it might not show up in the store search results.
It's not a decision we take lightly, and normally when we have done this, it's due to something game breaking. We're always shooting for wider compatibility though, so check back each update to see if your device has been added!