Gnutella.net file did exist for OS 9 & is found in the limewire preferences folder.
Location is: Macintosh HD -> System -> Preferences -> .limewire __ (notice the . in front of the limewire name)
If you view the Preferences folder in 'list' mode, you will find the .limewire folder sitting at or near the top of the preferences folder. Inside the LW preferences folder you will find the gnutella.net file which should be replaced; The link below will take you to a file hosting site where you can download a file named gnutella.net.
zSHARE - gnutella.net
If that link doesn't work, try this other one. gnutella.net

LW MUST be closed when you do this! This is very important.