If anyone is interested in the code, you can download the lua dissector script for wireshark. Copy it to the main directory of wireshark and add the line
dofile("gamespy.lua")to the end of the file init.lua (also located in the main directory of wireshark) . Note that's just a little script aimed at other programmers, feel free to improve.
 Aluigi, Explanation of the authentication method used by the GamespyCD-Key SDK 0.1.
 The Wireshark Wiki: Lua.