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 Deine Käufe auf einem neuen Gerät wiederherstellen.

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 Die iCloud Synchronisierung dauert bei mir sehr lange, oder funktioniert überhaupt nicht.

If you accidentally or intentionally disabled iCloud, you can learn how to iCloud Synchronisierung aktivieren und deaktivieren from this article.