I wouldn't worry for GDPR as the (guest) username is attached to personal details. If it shows with some content, e.g. post, that contains personal details then it would be up to them to request deletion. It would be difficult to prove the person requesting is the same person as the deleted account, as you would no longer have any evidence connecting them such as email or IP.
e.g.
- You delete user John Smith
- A person called John Smith contacts you and says you must delete the post where John Smith posted his phone number, address, date of birth, SS number and hat size.
- How do you know it is the same John Smith?