Gerillass 3.0.0 is out. One gradient mixin replaces two, and text-gradient takes its colours first, so read the migration guide before you upgrade.
Gerillass
v3.0.0Gerillass is released under the Apache License, Version 2.0. Copyright 2020 Halil İbrahim Çakıroğlu.
What that means in practice
You may use Gerillass in commercial and private work, modify it, and redistribute it. Two conditions come with that: keep the copyright notice and the licence text with any copy of the source you pass on, and state what you changed if you distribute a modified version.
The licence also grants patent rights from the contributors, which is the part that separates Apache 2.0 from MIT, and it offers no warranty.
None of this reaches the CSS you compile. What the library gives you is Sass that runs at build time; the stylesheet that comes out the other side is yours.
This page is a summary, not legal advice. The licence text is what governs.