Ultimatezip is one GPL software that you can use in windows to unzip tgz files
You will have to unzip the file twice anyway.
Any to zip a file and create a gz extension you have to tar the file first
and go for
Thats same as 'tar zcvf'
Why dont you just zip the folder using
zip -r folder
It will be compatible both in linux and in windows[can be unzipped using winzip]