Right now the GWebCache is just a way to get connected to the network. So when you start up your client it would contact a GWebCache instead of one of the hostcache servers. The speed ( in theory ) should be about the same as the current way but it is more decentralized. Right now there are only a handful of hostcache servers but there are many GWebCaches. Plus GWebCache servers run on web servers and use VERY little resources.

The future of the GWebCache may include features to help with downloads such as meshes and such but it currently does not.

AFAIK only shareaza and gnucleus support it. I think I remember reading that gtk-gnutella was also but I have never tried it.