Linking DMD D objects together with Cygwin GCC

[Simon]

I'm trying to link DMD D objects together with Cygwin GCC objects. I convert DMD OMF to COFF using link.exe -edit.
I use gphobos as runtime library.
Problem: When I declare a class in D, linking fails due to not resolved symbols like _Object7__ClassZ
I tried to convert DMD phobos lib to coff (using link.exe), but this failed at stream.obj, which had too many ?? sections. All other files went fine.
I must use the GCC linker, so what can I do to get the DMD phobos lib in COFF?