GDPR is a good thing to finally give more control of data back to end users, but like most births, it’s painful at first.
It is much about informing users what data you are storing, why you are storing it, and with whom you are sharing it.
This info should be simple and easy to understand, and presented at the point when you ask for the users consent.
The times of presenting long textwalls that users are supposed to read and automatically accept by using services are over.
Users need to choose to opt in, somthing that can not be default action.
You also need to give users a tool/option to delete all user data permanently, should they choose to do so.
Also, remember that the main issue is regarding sensitive info that can identify a person or information related to the person.
So about usernames, it should be easy enough to present a reasonable cause why you need to store it, and add an option in your app to delete user account.
I may have forgotten something but thats the big picture.