When using "gzip --decompress", the result is "gzip: MYFILE.zip: unknown suffix -- ignored" on Windows
You may be using the wrong tool.
gzip is file compressor/decompressor for files that generally have the file extension .gz and cannot extract files from a file like colorbox.zip. On linux you would use zip and unzip to add and extract files in a zip file. Maybe gnuwin32 has the same zip/unzip programs?