Hosts are cached in $GTK_GNUTELLA_DIR/{ultras,hosts}*. You cannot force it to connect only to a single vendor except to gtk-gnutella itself. You can however collect some addresses of BearShare peers and put it into the above mentioned files or connect manually to them. You can also put stable addresses into $GTK_GNUTELLA_DIR/whitelist, it will try to connect to those without intervention. Another option would be modifying the sources to drop all non-BearShare peers.

One feature you might want to disable during such experiments is "Monitor bad IP addresses". The whitelist can be edited on the fly and becomes effective after a few seconds. The others must be edited when gtk-gnutella is not running.