Hi everyone,
The intent is that the content generated by Rigify (including the rig ui script) belongs to the person who used Rigify to generate it. If Rigify being under GPL turns out to cause problems in that regard, I’m happy to change the license. But I don’t think it does cause problems with that.
You are absolutely allowed to sell rigs that you create with Rigify, including the generated scripts. Since the generate ui scripts use Blender Python API’s, they must be put under a GPL-compatible license if you distribute it. MIT, BSD, etc. are fine for that, I believe, if you want to make sure people can use it without restriction. And then the over-all rig can be under whatever license you like.
In short: do whatever you want, I’m not going to sue you anyway. My intent is that people be able to do whatever they want with the generated rigs.