The file you downloaded is a source file for compiling.

If you wanna try building it extract the folder somewhere.

Enter the directory and read the file README.Debian

The package names might be a little different for Ubuntu.

Just install Synaptic Package Manager and use the search option if you have to, synaptic can install the packages forgoing the apt-get install method.

The rest of the directions should work.

If it doesn't work thats still okay. It just means that the Debian rules don't apply to Ubuntu anymore.

Type

make clean
./build.sh

You should then be able to run the program without installing by executing the file gtk-gnutella that can be found in the "src" directory found in the folder you just did everything else in.