First you have to download Gnuplot as a ZIP file. The file name is gp400win32.zip or something similar, where 400 is the version number. Extract this ZIP file into the folder C:\Programme\gnuplot.
Ghostscript can be downloaded from the FTP mirror for Ghostscript (you need the EXE file). It is important to be administrator when executing this file!
On the homepage of ImageMagick you click on “Binary Releases” – “Windows”, download one of the EXE files and start is (as administrator if possible).