There are a few ways to earn Gems in the game as shown below. Weekly Resort Rating: tap on the rating board in front of the inn and craft orders to win prizes.
Sometimes you can win Gems.
The Sunken Temple: Sometimes you can win Gems by opening Treasure Chests. A few times a day you can open a Treasure Chest at the Sunken Temple for free. You can also pay coins or Gems to keep opening additional Treasure Chests!
Achievements: tap on the Inn to see the Achievements that will award you with Gems. Complete them and tap to collect your reward.
Purchase Gems: Tap on the "+" (plus sign) to the left of your current Gem total (upper left corner of your screen) to see the available offers.