There's an ongoing effort to write the GDL User's and Developer's guide. Currently the documentation covers an automatically-generated list of all available library routines and their keywords, about 100 code examples, and a few drafts of library routine descriptions and other chapters.
The documentation is available as a single PDF file (only in English): http://gnudatalanguage.sf.net/gdl.pdf [PDF, 1.4 MiB]
While GDL itself reached a beta status of development, the documentation is far from reaching an alpha status - help is very welcome! Please report any inconsistencies in the documentation as any other bugs in GDL. Please submit new additions to the text (incl. code examples) as patches to GDL (see support & feedback).
As GDL is almost 100% compatible to IDL (although not complete as many subroutines are waiting to be implemented), documentation and tutorials for IDL can also be utilized for GDL (see resources).