DecompressFile(Server.MapPath("~/Decompressed/FindEmai_onTextfile.zip"),Server.MapPath("~/Decompressed/FindEmai_onTextfile.txt"));

public static void DecompressFile(string sourceFileName, string destinationFileName)
    {

        FileStream outStream;

        FileStream inStream;

        //Check if the source file exist.          

        if (File.Exists(sourceFileName))
        {

            //Read teh input file

            inStream = File.OpenRead(sourceFileName);

            //Check if the destination file exist else create once

            outStream = File.Open(destinationFileName, FileMode.OpenOrCreate);

            //Now create a byte array to hold the contents of the file

            //Now increase the filecontent size more since the compressed file

            //size will always be less then the actuak file.

            byte[] fileContents = new byte[(inStream.Length * 100)];

            //Read the file and decompress

            GZipStream zipStream = new GZipStream(inStream, CompressionMode.Decompress, false);

            //Read the contents to this byte array

            int totalBytesRead = zipStream.Read(fileContents, 0, fileContents.Length);

            outStream.Write(fileContents, 0, totalBytesRead);

            //Now close all the streams.

            zipStream.Close();

            inStream.Close();

            outStream.Close();

        }

    }