If you've been directed here from the game, you're seeing the 'Sync Alert!' because there are two possible save choices for your Garden.
Please check the Garden Level/Gems progress shown in the pop-up. The higher level number would typically be the game with the most progress made. However, a good general practice would be to choose the progress from the device that you last played on. If you last played on your current device, choose the save on the left, and if you last played on a different device, choose the cloud save on the right.
If you are in any doubt which is the correct save or you have further questions, then please contact our support team via MergeGardens@Futureplaygames.com
If you would like to sync using your Facebook account, then please go to the in-game Settings and tap on Cloud Save to see the options available.