I don't have libgmp-3.dll, and every attempt I make to get it results in
what would seem to be an incompatible version of the DLL, producing the
message: "The procedure entry point __gmp_get_memory_functions could not be
located..."
The version of gmp distributed with the current version of mingw is
libgmp-10.dll. Older packages don't appear to be compatible.

Consider dropping the libgmp-3.dll you build against in the bin/ directory alongside gdc.exe?