How I can Own a inactive gitlab username?

I wanted to own an inactive username. How I can request this?

You have to write to the Gitlab.com support. They have a quite clear policy:

User namespaces can be reassigned if both of the following are true:
    The user's last sign in was at least two years ago.
    The user is not the sole owner of any active projects.

Group namespaces can be reassigned if one of the following is true:
    There is no data (no project or project(s) are empty).
    The owner's last sign in was at least two years ago.