yes.

greenstone comes with a built-in z39.50 client, although none
of our distributed binaries include it. You will have to build
from source, and configure with the command
./configure --enable-z3950
on a unix-like (linux, solaris, mac os x, etc) machine.
There is a config file in <gsdl>/etc/packages/z3950.cfg that
specifies which z39.50 servers to connect to.

It should theoretically be possible to compile under windows as well,
although we've never tried it, and it would require some fiddling with
the makefiles, etc.

There was also an experimental z39.50 server interface written for
greenstone, although that was never released and is no longer maintained.

Hope this helps
John McPherson