The GTI zaust wont bolt straight on as the driver bore is smaller, and the back box has a pipe going all the way to the front silencer rather than to a rear silencer like on the GTI.
You have 3 choices:
1. Fit a 1.8 GL/Driver downpipe, this will mate up to your manifold and allow you to fit a GTI zaust from the centre silencer back
2. Fit a complete GTI exhaust from the manifold back
3. Get a GTI rear silencer & backbox, then get a garage to make up an adaptor so it will fit to the 1.6 front silencer