There is link on generated gapi namespace documentation (content will be updated after fixes).

Issues:

• broken layout after/in BGR2Gray entry
• navigation header doesn't look correct too (grouping issues):
  G-API framework » G-API Graph Compilation ArgumentsG-API framework » G-API Data Types » | G-API Metadata Descriptors
• perhaps we need to ignore in documentation entries with std::enable_if< std::is_same< T, GCompileArgs >::value, GCompileArgs >::type (or move it into nested group through @ingroup)