Main advantage would be running existing IDL programs without the proprietary license. For any new development the alternatives you mention have a larger userbase.
Octave is also open source. It's GNU Octave, in fact
GNU Octave is intended as a non-proprietary replacement for MATLAB not for IDL, so, as another poster has written above, both these 2 GNU projects have the purpose of replacing certain proprietary tools, but they are intended for different user bases, who were previously familiar with those 2 different proprietary tools.
IDL is a much older established language dating back to the '70s. So GDL would appeal to anyone needing to run or develop IDL code but wishing to avoid paying the license fee.
I remember using IDL to analyze atmospheric observations made by satellites back in 1991.