New in iOS 14 (not previous versions) is the ability for apps to let members share their subscription. This means anyone in your Family Sharing group can use Gym Plan without any additional cost while the main Organiser has an active subscription.
Here's a link to the Apple help center regarding Family Sharing of subscriptions.
- Have Family Sharing set up and all the members be part of the same family.
- In Settings / Apple Id (the first item in the Settings app) / Family Sharing / Purchase Sharing / Share Purchases with Family -> Turned On.
- In App Store app / Account / Subscriptions / Gym Plan / Share with Family -> Turned On.