TAR
files are just one big file of all your other files appended together,
GZ
just compresses that 1 file, so you have to decompress all the file to extract what you want (up to the start of the file you want of course) so you can't just extract the file you want since the format does not allow it.
Try using a component like :
http://icsharpcode.github.io/SharpZipLib/[
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