From GDL Wiki
GDL - GNU Data Language
GDL is a free IDL (Interactive Data Language) compatible incremental compiler (i. e. runs IDL programs). IDL is a registered trademark of ITT Visual Information Solutions.
Compiling - Source code instructions for various platforms.
Tutorials - Tips and tricks for GDL.
Routines - List of IDL 6.1 routines implemented in GDL (maintained by Alain Coulais).