As nearly always under Linux you will find the executables either under /usr/bin or /usr/local/bin. You may find it either with 'which gtk-gnutella', 'find / -name "gtk-gnutella" 2>/dev/null' or, if the locate package is installed, with 'locate gtk-gnutella'.

Most of the time it's within the path, so a 'gtk-gnutella &' in a X-based console will do the job.

Viele Grüße,
Christian