It must be under GPL . Because WordPress is under GPL and plugins are using WordPress functions, classes etc.
They ask people to release them under GPL v2+ but that doesn´t mean all the authors do that. From their plugin page:
It is customary to follow the standard header with information about licensing for the Plugin. Most Plugins use the GPL license used by WordPress or a license compatible with the GPL . To indicate a GPL license, include the following lines in your Plugin: