Just a comment.  I compiled GDL 0.8.8 on my Linux 2.4.29 with gcc 3.3.4 only to run into an error at gdlgstream.cpp.   The error was that RGB_HLS was undefined.

Now this comes from plplot.h, but for my version of plplot 5.5.0 there is no RGB_HLS, but there is a plRGB_HLS (and plHLS_RGB).

You need to edit the gdlgstream.hpp file and add a line:

#define RGB_HLS plRGB_HLS
(I also added #define HLS_RGB plHLS_RGB just in case).

Hopefully a developer reads this.