The host cache pane provides information about other nodes seen on the GnutellaNet. They are not necessary connected at the moment. They have been seen at some time in the past. The host cache is used when new connections are needed. This is not only for your version of gtk-gnutealla, but any other node that might contact you will be given random values from the host cache.
The buttons at the bottom allow you to clear the caches. However, you should never do this. If none of the hosts work, gtk-gnutella will still use other means to find good hosts. It is only sensible to clear these values if you haven't used the program in several months. It is useful to adjust the number of hosts stored in the cache, but the defaults work very well. For the most part, after a single connection is made, new hosts will come directly from the network.