Stream stream = null;
if (loadFromResources == true)
{
    TextAsset textAsset = Resources.Load(fullPath) as TextAsset;
    Debug.Log(textAsset);
    stream = new MemoryStream(textAsset.bytes);
}
else
{
    FileInfo fi1 = new FileInfo(fullPath);
    if (fi1.Extension.Equals(".gz"))
    {
        stream = new MemoryStream();
        byte[] buffer = new byte[4096];

        using (Stream inGzipStream = new GZipStream(File.Open(fullPath,FileMode.Open), CompressionMode.Decompress))
        {
            int bytesRead;
            while ((bytesRead = inGzipStream.Read(buffer, 0, buffer.Length)) > 0)
            {
                stream.Write(buffer, 0, bytesRead);
            }
        }

    }
    else
        stream = File.Open(fullPath, FileMode.Open);
}
using (BinaryReader reader = new BinaryReader(stream))
{
    //header headerKey substruct:
    headerKey.sizeof_hdr = reader.ReadInt32(); //ERROR
}