As GDL is almost 100% compatible to IDL (although not complete as many subroutines are waiting to be implemented), resources for IDL can also be utilized for GDL.

Many IDL resources can be found on the page run by IDL guru David Fanning.

Lots of routines written for IDL can be found with the IDL Libraries Browser many of them should also work with GDL.

An IDL online tutorial is available at the Scientific Computing and Visualization website.

For writing programs in GDL using Emacs the Emacs addon (mode) IDLWAVE, maintained by J.D. Smith is highly appreciated.