I've just finished making a controller for a KotH challenge, and I wanted to know: Can I use it for the KotH and license it under the GNU GPL v3, or does it have to be licensed with the MIT License to be used for a KotH on CGCC?
You can license it however you wish offsite
The important (but still not necessary) part is that people can run it. Preferably the source should be shared for easier testing and for malware protection.
On-site, code is licensed under CC-BY-SA 4.0
This applies to controllers you post in the question, but it's probably not a good idea to put it there (e.g. if the controller includes the code for the submissions, which is bad for the same reason as leaderboards).