Fweeb writes:
A lot of folks who make Blender add-ons are a little unclear on what you need to do in order to make sure that the add-on properly complies with the GNU General Public License (otherwise know as the GPL... it's the same license that Blender itself uses, and, yes, add-ons must be compatible with that license). So to help clear any confusion, we wrote this comprehensive guide on what exactly you need to do in order to make sure that your add-on is fully on the up-and-up with respect to the terms of the GPL.
So here you go: everything you wanted to know (and probably a few things you didn't) about making your Blender add-ons GPL-compliant.
https://orangeturbine.com/posts/how-to-make-your-blender-add-on-gpl-compliant