By your description, your talking about the slave cylinder.
It's been a while since I had a slave off but I don't believe there is a gasket between the slave and the case.
The leak is most likely either from the banjo bolt #12 or the interior seal of the slave #15
Clean the area real good and watch for new leaking.
http://www.partzilla.com/parts/search/Honda/Motorcycle/1999/GL1500CF+A/CLUTCH+COVER/parts.html