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/Gem progress shown; the higher number of Garden Level/Gems would typically be the game with the most progress made.
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
This save option should mean that your progress is saved to the device that you are using. This method cannot be used for transferring your progress from one device to another - for that you would also need to contact support.