What is the cmatrix license...?

Headers state GPL-3+:

    cmatrix is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

While the readme and COPYING suggest GPL-3.

From what I gather there is no formal "GPL-3-or-later" license and the note in the file header defines that.

The licensing situation is unclear so I please ask @abishekvashok for clarification.