You can use GymYou on as many devices as you want. If you have purchased a GymYou Pro subscription, you can restore your purchases on each device, sharing the same Apple ID.
Learn how to do that in the article Restore your purchases on a new device.

iCloud sync is enabled by default. If you have not disabled it manually, your data should be synced across all of your devices automatically. Be aware that it may take some time to initially sync all your data if you installed a fresh copy of GymYou. If you are experiencing issues with iCloud sync, please check the article My iCloud sync takes a long time or does not work at all.

If you accidentally or intentionally disabled iCloud, you can learn how to Enable and Disable iCloud sync from this article.