Add license

[Miepee]

Currently this project is missing a license, which makes the consequences of copying, distributing, or modifying gz uncertain.

See this for more information:
https://choosealicense.com/no-permission/

[neonfuz]

I second this, though I would also like to point out that in order to change the current code's license you need all 18 contributors to agree. Should do this before accepting any more contributors.

[Miepee]

Not a lawyer, but something that might be helpful nonetheless: as far as I'm aware, you don't need all 18 contributors approval, but only the approval of the contributors whose code can currently be found.
If there was code from user foo, which has been been fully replaced/refactored by a user bar, then the approval of foo would not be required.
Mentioning this because in the contributors list, I can see quite a few people who only contributed smaller lines of code.