GNU Radio documentation can be found in several places throughout the internet. Most “user” level documentation such as block usage, tutorials, example applications, and guides can be found in the GNU Radio Wiki.
A great place to start for someone totally new are the Guided Tutorials.
For someone with basic GNU Radio experience, but looking to create their own blocks and become an expert, we highly recommend going through each page in the Usage Manual.
For API documentation please consult the Doxygen documentation.