6.1.0 has a bug that can, if everything goes wrong just right, delete files in the root of the project. I only saw it happen when running the test suite for Gut and only the test_doubler.gd test script. I don't recall ever seeing it happen in my own game, but just to be safe you should upgrade.
I've started a 3.1 branch that I will be keeping inline with master. I've gotten it to run but there are issues. Check open issues and the 3.1 wiki page.
GUT (Godot Unit Test) is a utility for writing tests for your Godot Engine game. It allows you to write tests for your gdscript in gdscript.
More info can be found in the wiki.
Version 6.0.0 is Godot 3.0 compatible. These changes are not compatible with any of the 2.x versions of Godot. The godot_2x branch has been created to hold the old version of Gut that works with Godot 2.x. Barring any severe issues, there will not be any more development for Godot 2.x.
Gut is provided under the MIT license. License is in addons/gut/LICENSE.md
Here's a couple wiki links to get you started.