What file did you download?
If you have downloaded the .rpm file the process if quite easy. Just do:

rpm -Uhv name_of_rpm.rpm

If you have the sources you have to extract and compile them. You may need to install additional software to compile it (like libxml2 or gtk devel headers):

tar xfvz name_of_archive.tar.gz
cd gtk-gnutella
./configure
make
su -c "make install"

does this work for you?
Now after you have installed the RPM or installed the sources, just type "gtk-gnutella" on your shell prompt.
Or if you use kde, type ALT+F2 and type it there