In the Display/Highlight/Sources there is already an IDL option, BUT this is for LibIDL and NOT for IDL. Here is a patch for Gedit allowing user to highlight some IDL (RSI/ITT) source codes. This patch is not exhaustive, this is why any comments are welcome. You just have to copy the patch into $path/gtksourceview-2.0/language-specs/ and run Gedit :

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A.Lucas / IPGP / 09-2009