It should be possible to either compile the C code as a DLL and call it from a C# application using PInvoke, or else incorporate the methods directly into your C# app.
Once you have the GSM data decoded into sample data, writing it out to a WAV file is very simple.
Best Answer
You could use FFMPEG called from the command-line, like so:
NOTE: Untested
Also, Windows binaries are available (I had some from another place as well) here.