Hey Giah,
Please forgive the top-post as I am writing this on my phone.
C#, as you may know, uses the common language runtime. There is no "neat" objective c way to do this, so you'll have to fall back on C.
You could use something from the following article on stackoverflow:
http://stackoverflow.com/questions/4428267/calling-c-sharp-from-cOr maybe this...
http://www.codeproject.com/Tips/695387/Calling-Csharp-NET-methods-from-unmanaged-C-CpluspI've never tried this so there is some question if any of these methods would work given that GNUstep is compiled with gcc or clang on Windows.
Unsurprisingly there are very few resources to figure out how to do this given that most calls go the other way from C to C#.
One other thing you might try is exposing the calls on the C# side as COM objects and then writing code to call those objects in C on the Objective-C side. This might actually be the cleanest solution.
Let us know what you decide. I hope this helps even a little.
Thanks. GC