You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.
The author has dedicated Gridmapper to the public domain by waiving all of his rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
The legal text: CC0 1.0 Universal, the Public Domain Dedication (CC0 1.0).
Some context: Public Domain Software.
