May I release my source code under the GPL?

I have troubles finding a list of licences compatible with Unity3D as the “license” keyword often brings up things about the Free/Trial/Pro licences of Unity itself.

I would like to release the source code of my game under the latest GPL and all its artwork under CreativeCommon BY-SA-NC. Is that possible with Unity?

I am not using anything from the Asset Store, not even standard assets such as the character controllers.

EDIT: Argh, I just noticed that Unity’s UI code is released under the MIT/X11 license. I created custom LayoutGroup components that are 50% copy-paste from the original ones. That’s a mess.

Yes, you can release your own code under GPL. There already are many projects, scripts and assets released under GPL.

Just make sure you do not include anything that will infringe the copyrights of anyone else.