From gnu.org:
If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins.
Thusly, just as if I were to write a GPL plugin, you have the right to take it and modify the plugin. However, your modified version is still GPL, otherwise you wouldn’t have the right to modify it in the first place.
Thusly, if WordPress is GNU/GPL, and a plugin is part of that program, the plugin should be GNU/GPL as well.
