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.