Answer:
Tragedy of the commons.
Step-by-step explanation:
The "Tragedy of the Commons" refers to the phenomenon where People overuse a common resource. It is a situation arise when individual user share resources with much other and demand increase in comparison to the supply of resources, which lead to depletion, destruction, or damage to resources due to overconsumption. It can be prevented by using certain measures like fixing ownership of resources, assigning basic rules and regulations for usage of resources, penalizing for damage, etc.
In the given case, a retired athlete built a gym and made it common and free for neighborhood residents, which lead to overuse of gym facility and that is a tragedy of common arise.