I'm not sure the exact amount but it's not very much at all I'm afraid so if it's already been done before you may need to look at a thicker HG as I've read of that over there though I'll have a search on the 6 forum and see what I can find.
http://www.306gti6.com/forum/showthread.php?id=148223
Theres a few other posts on the thread worth reading but they seemed to be the key two for this and anyone else in the future.
http://www.306gti6.com/forum/showthread.php?id=148223
Quote:...
There is a small notch at one end which has rough casting in it. There will be a "step" up from this onto the gasket face. Once this "step" disappears there is no more meat on the head to skim off....
Quote:To see if the head has been skimmed previously measure its total height less the cam ladders, standard is 137mm +/- 0.05.
minimum height is 136.8mm +/- 0.05.
i.e 0.2mm.
standard gasket is 1.2mm, Pug is the cheapest place for the standard MLS one, GSF only a few quid more for a Victor Reinz one, avoid ultraparts as they have the rivets on the end of the gasket which interferes with the timing belt tensioners.
Theres a few other posts on the thread worth reading but they seemed to be the key two for this and anyone else in the future.