This might be a false alarm, because I think that GDCM configure should never be calling FindLibPROJ in the first place. It looks like GDCM configure is invoking parts of VTK CMake configure for no reason that I can understand.
The correct place for PROJ linkage should be embedded in the VTK binary library file, not in any CMake files produced by the VTK build. This is proper encapsulation. I might be wrong, I am very unfamiliar with CMake details.
So the short answer is, don’t spend your time looking for this ghost, unless I come up with something more specific. I need to take a closer look at the GDCM build system.