The provided Win32-version unfortunately doesn't support German Umlaute (UTF8 is missing in GTK 1.x), the UNIX-version does.
For compiling the source-code you'll need GtkAda and an Ada-Compiler, i.e. GNAT.
Download the source here.
The Win32-executable (1.3MB) is a zipfile. Uncompress it to a directory and run rot13 from it. The dlls are needed because of GtkAda.
You may also want a command-line-only version (for both, Windows and UNIX), so take this onefile-sourcecode and compile it.