Many ISPs have been known to filter or shape peer-to-peer traffic. The Azureus wiki gives an extensive list of ISPs known to filter or shape BitTorrent traffic. As can be seen from this discussion many of the same ISPs are shaping Gnutella traffic.
gtk-gnutella was the first Gnutella client to provide encryption to prevent traffic filtering and shaping of Gnutella traffic. If you have an ISP that you know is detecting Gnutella traffic, you can set the tls_enforce parameter. The tls_enforce parameter is set from the "Debugging" tab of the Preferences dialog (control-P). Type tls_enforce in the "Property pattern" and click on the value "FALSE" and change it to "TRUE".
If your ISP is not shaping traffic, there is no need to set the tls_enforce parameter. gtk-gnutella will detect if other servents wish to use encrypted connections. gtk-gnutella must be compile to use the GnuTLS libraries for the encryption to work.