The easiest way to get gtk-gnutella is to install it as part of your distribution. Most distributions carry gtk-gnutella pre-packaged these days. This is not always the best option, though. Since the Gnutella sometimes undergoes significant change, and since buggy clients may cause many problems to the Gnutella network, gtk-gnutella clients are coded with a maximum lifetime. After a pre-determined period gtk-gnutella will start to complain and eventually will stop working. Distributions are not always quick to pick up new versions because stability is important to them, so you may be installing a version of gtk-gnutella that is close to or already past its limited lifetime. This is true in particular if you install from offline media such as DVD.
Some web sights offer gtk-gnutella packages. RPM find and Linux App finder have packages for some popular Linux distributions. Also, the gtk-gnutella web site may contains some packages.
A better way to get gtk-gnutella is to install it yourself. The gtk-gnutella web site contains the source archive which allows you to compile gtk-gnutella yourself. You can also use the web site to check if your currently installed version is still the most recent version; gtk-gnutella will also notify you if it sees a newer version on the gnutella network.