I just wanted to give another update on what I've been working on for the past few days. GNUstep-corebase can now be built using only the Windows SDK and no longer requires -make, -base or libobjc. I added an nmake makefile to the Source/ folder. If you want to test this out you will need:
To build, the icu/ directory must be at the same level the corebase/ directory. Open the SDK's CMD shell, cd into corebase\Source and type "nmake -f Makefile.nmake". You'll end up with gnustep-corebase.{lib,exp,dll}. Copy these 3 files, plus the icu dlls (located in icu\bin{64,}) to any directory you want. At this point you should just be able to use "cl" and "link" to compile any software you want. By modifying CFTesting.h in the Test/ dir, I was able to successfully build and run some of the tests in the test suite.
What's missing: - Anything ObjC related - The CF-types that are simply wrappers around ObjC objects (CFBundle, CFPropertyList, CFURL, etc)
This is just a proof of concept, so expect problems.